Allow generic django_management commands

This commit removes the restriction on django management commands.  If a command is unknown to the django installation, there will be a concise error produced.

for example:

  tasks:
    - name: invalid command
      django_manage: virtualenv="/valid/virtualenv" app_path="/valid/app_path" command="nowaydude"

Results in:

failed: [hostname] => {"cmd": "python manage.py nowaydude", "failed": true}
msg: stdout: Unknown command: 'nowaydude'
Type 'manage.py help' for usage.

:stderr: Unknown django command: nowaydude