Only trigger provider when consumer child present

In some cases we are unneccesary running content provider job even
when no functional jobs are triggered. Example[1].

Adding files filter which is present in child functional jobs in the
content-provider job as well , So that it will only trigger when
functional jobs trigger.

Also updating filter -  tripleo_container_image_prepare to
tripleo_container_image_prepare_default as in CI we utilize
the tripleo_container_image_prepare_default role[2]

[1] https://review.opendev.org/c/openstack/tripleo-operator-ansible/+/793722/
[2] https://opendev.org/openstack/tripleo-quickstart-extras/src/branch/master/roles/standalone/tasks/containers.yml#L92

Change-Id: Ic83a3296a1a1ae882060aebb271112635368b9d4

zuul.d/layout.yaml

@@ -12,6 +12,11 @@
               - openstack-tox-pep8
               - openstack-tox-py36
               - openstack-tox-py37
+            files: &content_provider
+              - ^roles/tripleo_undercloud_install/.*$
+              - ^roles/tripleo_undercloud_minion_install/.*$
+              - ^roles/tripleo_deploy/.*$
+              - ^roles/tripleo_container_image_prepare_default/.*$
         - openstack-tox-docs: &tripleo-docs
             files:
             - ^doc/.*
@@ -60,11 +65,12 @@
             dependencies: *deps_content_provider_c8
             files: &standalone_files
               - ^roles/tripleo_deploy/.*$
-              - ^roles/tripleo_container_image_prepare/.*$
+              - ^roles/tripleo_container_image_prepare_default/.*$
     gate:
       jobs:
         - openstack-tox-linters
-        - tripleo-ci-centos-8-content-provider
+        - tripleo-ci-centos-8-content-provider:
+            files: *content_provider
         - openstack-tox-docs: *tripleo-docs
         - tripleo-ci-centos-8-containers-multinode: *job_containers_multinode
         - tripleo-ci-centos-8-containers-undercloud-minion: *job_containers_undercloud_minion