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'
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        /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'
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        script/delayed_job:5:in `<main>'
Merge pull request #2428 from xsuchy/pull-req-task_statuses-organization_id

allow to be task_statuses.organization_id null