afs-client: move reduced cache to group variable

For gate testing we need the smaller AFS cache size applied to
everything that might install openafs, not just the mirror nodes.
Move the definition to the afs-client group.

Change-Id: Id27efd2f12f5ac3f351f65fa1ae513624a53df90

playbooks/zuul/run-base.yaml

@@ -83,7 +83,7 @@
         - group_vars/review.yaml
         - group_vars/review-dev.yaml
         - group_vars/control-plane-clouds.yaml
-        - group_vars/mirror_opendev.yaml
+        - group_vars/afs-client.yaml
         - host_vars/bridge.openstack.org.yaml
         - host_vars/letsencrypt01.opendev.org.yaml
         - host_vars/letsencrypt02.opendev.org.yaml