allow to be task_statuses.organization_id null

When organization is going to be deleted, the org record is deleted from db,
and is created async task, which belongs to org which just have been deleted.

Task which is deleting organization will have organization_id set to null.

addressing:
```
[ERROR 2013-06-04 05:15:04 app  #5643] <ActiveRecord::InvalidForeignKey> PGError: ERROR:  update or delete on table "organizations" violates foreign key constraint "task_statuses_organization_id_fk" on table "task_statuses"
DETAIL:  Key (id)=(5) is still referenced from table "task_statuses".
: COMMIT
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/connection_adapters/postgresql_adapter.rb:654:in `async_exec'
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/connection_adapters/postgresql_adapter.rb:654:in `block in execute'
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/connection_adapters/abstract_adapter.rb:280:in `block in log'
        /opt/rh/ruby193/root/usr/share/gems/gems/activesupport-3.2.8/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/connection_adapters/abstract_adapter.rb:275:in `log'
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/connection_adapters/postgresql_adapter.rb:653:in `execute'
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/connection_adapters/postgresql_adapter.rb:708:in `commit_db_transaction'
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/connection_adapters/abstract/database_statements.rb:217:in `transaction'
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/transactions.rb:208:in `transaction'
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/transactions.rb:293:in `with_transaction_returning_status'
        /opt/rh/ruby193/root/usr/share/gems/gems/activerecord-3.2.8/lib/active_record/transactions.rb:236:in `destroy'
        /usr/share/katello/app/models/glue.rb:92:in `destroy'
        /usr/share/katello/app/models/organization_destroyer.rb:39:in `run'
        /usr/share/katello/app/models/async_operation.rb:58:in `perform'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/backend/base.rb:94:in `block in invoke_job'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:60:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:60:in `block in initialize'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:65:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:65:in `execute'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:38:in `run_callbacks'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/backend/base.rb:91:in `invoke_job'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:178:in `block (2 levels) in run'
        /opt/rh/ruby193/root/usr/share/ruby/timeout.rb:68:in `timeout'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:178:in `block in run'
        /opt/rh/ruby193/root/usr/share/ruby/benchmark.rb:295:in `realtime'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:177:in `run'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:234:in `block in reserve_and_run_one_job'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:60:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:60:in `block in initialize'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:65:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:65:in `execute'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:38:in `run_callbacks'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:234:in `reserve_and_run_one_job'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:162:in `block in work_off'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:161:in `times'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:161:in `work_off'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:133:in `block (4 levels) in start'
        /opt/rh/ruby193/root/usr/share/ruby/benchmark.rb:295:in `realtime'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:132:in `block (3 levels) in start'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:60:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:60:in `block in initialize'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:65:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:65:in `execute'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:38:in `run_callbacks'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:129:in `block (2 levels) in start'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:128:in `loop'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:128:in `block in start'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/plugins/clear_locks.rb:7:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/plugins/clear_locks.rb:7:in `block (2 levels) in <class:ClearLocks>'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:78:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:78:in `block (2 levels) in add'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:60:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:60:in `block in initialize'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:78:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:78:in `block in add'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:65:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:65:in `execute'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/lifecycle.rb:38:in `run_callbacks'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/worker.rb:127:in `start'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/command.rb:101:in `run'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/command.rb:89:in `block in run_process'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons/application.rb:249:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons/application.rb:249:in `block in start_proc'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons/daemonize.rb:197:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons/daemonize.rb:197:in `call_as_daemon'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons/application.rb:253:in `start_proc'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons/application.rb:293:in `start'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons/controller.rb:70:in `run'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons.rb:195:in `block in run_proc'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons/cmdline.rb:109:in `call'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons/cmdline.rb:109:in `catch_exceptions'
        /opt/rh/ruby193/root/usr/share/gems/gems/daemons-1.1.4/lib/daemons.rb:194:in `run_proc'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/command.rb:87:in `run_process'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/command.rb:80:in `block in daemonize'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/command.rb:78:in `times'
        /opt/rh/ruby193/root/usr/share/gems/gems/delayed_job-3.0.2/lib/delayed/command.rb:78:in `daemonize'
        script/delayed_job:5:in `<main>'
```