engine: Remove gluster hooks from db when no servers in cluster

If the host being removed is the last server in the cluster
clean up the entries for gluster hooks as well.

Change-Id: Ib4f99798cb4d0ebe70045c20286939f54d6b1fcc
Bug-Url: https://bugzilla.redhat.com/986171
Signed-off-by: Sahina Bose <sabose@redhat.com>