Fix prometheus-alertmanager cluster bug

Edited the
ansible/roles/prometheus/templates/prometheus-alertmanager.json.j2 file
to change the mesh.peer and mesh.listen-address to cluter.peer and
cluster.listen-address.  This stopped alertmanager from crashing with
error "--mesh.peer is an invalid flag"

Change-Id: Ia0447674b9ec377a814f37b70b4863a2bd1348ce
Signed-off-by: Mark Flynn <markandrewflynn@gmail.com>