Keep creating keystone admin endpoint for heat

Heat still depends on an keystone admin endpoint being present although
it has no functional difference anymore from the other endpoints with the
v3 API. Make devstack create one.

Change-Id: Icd786e89aba5b593feee165226010b25f8627efd

.zuul.yaml

@@ -123,6 +123,8 @@
       devstack_localrc:
         DISABLE_AMP_IMAGE_BUILD: true
         Q_SERVICE_PLUGIN_CLASSES: qos,trunk
+        # TODO(frickler): drop this once heat no longer needs it
+        KEYSTONE_ADMIN_ENDPOINT: true
       devstack_plugins:
         heat: https://opendev.org/openstack/heat
       tox_environment: