Trigger bifrost CI job on changes and periodically

Use Zuul's filtering mechanisms to only run the bifrost CI job in the
check pipeline when changes are made to the bifrost role. This should
reduce load on the CI system.

Also adds a periodic pipeline containing the bifrost job.

Change-Id: Ie7293d40616c91d2376f4cc85a8e87a130607d99

.zuul.yaml

@@ -8,12 +8,16 @@
         - kolla-ansible-ubuntu-source-ceph
         - kolla-ansible-centos-source-ceph
         - kolla-ansible-oraclelinux-source-ceph
-        - kolla-ansible-bifrost-centos-source
-
+        - kolla-ansible-bifrost-centos-source:
+            files: ^ansible\/roles\/bifrost\/.*
         - openstack-tox-lower-constraints
     gate:
       jobs:
         - openstack-tox-lower-constraints
+    periodic:
+      jobs:
+        - kolla-ansible-bifrost-centos-source
+
 - nodeset:
     name: kolla-ansible-centos
     nodes: