CI: enable ceilometer plugin

This should ensure Gnocchi is installed, otherwise we get the following
error in ck-proc:

    keystoneauth1.exceptions.catalog.EndpointNotFound: internalURL endpoint for metric service in RegionOne region not found

Change-Id: Ie0fe899caac481ea31675fcc17e6e849689bde77

Author: priteau

.zuul.yaml

@@ -7,6 +7,7 @@
     run: playbooks/cloudkittyclient-devstack-functional/run.yaml
     post-run: playbooks/cloudkittyclient-devstack-functional/post.yaml
     required-projects:
+      - name: openstack/ceilometer
       - name: openstack/cloudkitty
       - name: openstack/python-cloudkittyclient
     roles:
@@ -18,6 +19,7 @@
       - ^releasenotes/.*$
     vars:
       devstack_plugins:
+        ceilometer: https://opendev.org/openstack/ceilometer
         cloudkitty: https://opendev.org/openstack/cloudkitty
       devstack_localrc:
         CLOUDKITTY_FETCHER: keystone