Rollback exception. Fixed bug with deployment description in hosts section.

Fixed bug with env-configuration where hosts section containing deployment description.
Added *delete-on-rollback* option in general and env-deployment section

Bug with next case:
```
hosts:
  123.123.123.123:1234:
    tasks:
      on-deploy:
        - someTask1
        - someTask2
  123.123.123.124:1234:
    deployment:
      strategy: disabled
      user: someuser
      identity-file: /path/to/ssh/private/key
    tasks:
      on-deploy:
        - someBuiltIn/some-built-in-task
```

Now we can determine whether to delete the current release directory through the parameter:  *delete-on-rollback*. In general.yml or in deployment-section in environment config file.

**!UPD**:	added restriction to run the rollback. Now if deploy process running - rollback works only at post-release and post-deploy stages.