Fix issues with importing the Login form

The Login form lives in openstack_auth.forms and should be directly
imported from that file.

Change-Id: I42808530024bebb01604adbf4828769812856bf3
Closes-Bug: #1332149
(cherry picked from commit 345ccc9d503e6e55fe46d7813958c0081cc1cffe)
Merge branch 'stable/havana' of https://github.com/openstack/horizon into stable/havana_routerrules