Rename AUTH_PLUGINS option
Rename AUTH_PLUGINS option

When I redeployed horizon using the newer plugin interfaces I noticed
that horizon already had the options AUTHENTICATION_BACKENDS and
AUTHENTICATION_URLS.

AUTH_PLUGINS is inconsistent here and we should update it to
AUTHENTICATION_PLUGINS. There has not been a release with this variable
to worry about.

Change-Id: Idab7770a9e61979fea15db63764423414665ba49

dan1/django_openstack_auth_proto,dan1/django_openstack_auth_proto,redhat-openstack/django_openstack_auth,redhat-openstack/django_openstack_auth