Replace UserManager/TenantManager with None in tests

UserManager/TenantManager seems private classes in keystoneclient
and the interface was changed in keystoneclient 0.10.0.
django-openstack-auth tests actually don't need these managers,
so this commit makes test data not use these classes.

Change-Id: I9398c705acab262e89d4801da84ccea188980d99
Closes-Bug: #1349485