fixed minor bug

fixed bug in initial vhost deploy

ERROR    An error occured
Traceback (most recent call last):
  File "/usr/lib/python3.3/site-packages/Imp/agent/__init__.py", line 433, in deploy_config
    provider.execute(resource)
  File "/usr/lib/python3.3/site-packages/Imp/agent/handler.py", line 128, in execute
    changed = self.do_changes(resource)
  File "./libs/rabbitmq/plugins/__init__.py", line 114, in do_changes
    changes = self.list_changes(resource)
  File "./libs/rabbitmq/plugins/__init__.py", line 109, in list_changes
    current = self.check_resource(resource)
  File "./libs/rabbitmq/plugins/__init__.py", line 102, in check_resource
    current.users = [p["user"] for p in permissions]
TypeError: 'NoneType' object is not iterable