Docstring cleanup for return type
Docstring cleanup for return type

The :returns: directive doesn't take an argument. To specify the
return type, use the :rtype: directive.

Change-Id: I3aaab824792333b3f75a10af92f5b712cc9b4ff6

citrix-openstack-build/keystoneauth,sileht/keystoneauth,jamielennox/keystoneauth