Add capability for Keystone V3 Authentication.

For multi-domain model, set OPENSTACK_KEYSTONE_MULTIDOMAIN_SUPPORT
to True and the login form will prompt the user for Domain name.

For single-domain model, set OPENSTACK_KEYSTONE_MULTIDOMAIN_SUPPORT
to False. The application will use the name of the default domain in
OPENSTACK_KEYSTONE_DEFAULT_DOMAIN to login.

Cleanup the unused Tenant field in the login form.

Implements blueprint login-domain-support