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.

Dependency: https://review.openstack.org/#/c/21942/

Implements blueprint login-domain-support