openafs-server: ensure vos_release keys installed on new servers

Add the afs-1.8 group in so that the vos release keys get installed.
Doesn't make a difference now when all servers already have the keys,
but would if a server is re-created.

Change-Id: Ic3902170d29ada1d23a446cc4d7eec39b371f733

playbooks/service-afs.yaml

@@ -26,7 +26,7 @@
       register: vos_release_keypair
 
 # Note: relies on vos_release_keypair installed to mirror above
-- hosts: "afs:!disabled"
+- hosts: "afs:afs-1.8:!disabled"
   name: "Allow remote vos_release"
   roles:
     - vos-release