Update mcapi is_admin method

MCAPIManager removes one level of the api json response during processing.

The api response is of the form:

  {'DATA': {'is_admin': False|True }}

This update extracts the result dictionary from the response.

MCAPIManager does not work if the response is not of the form above.

Change-Id: I242a604d62b9bf99516978e44aa7a94580977b96
Fixes: OPENSTACK-694