Configure ceph dashboard backend and dashboard_frontend_vip

Configure ceph dashboard backend and dashboard_frontend_vip

This change introduces a new set of tasks to configure the
ceph dashboard backend and listen just on the mgr related
subnet (and not on '*'). For the same reason the proper
server address is added in both prometheus and alertmanger
systemd units.
This patch also adds the "dashboard_frontend_vip" parameter
to make sure we're able to support the HA model when multiple
grafana instances are deployed.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1792230
Signed-off-by: Francesco Pantano <fpantano@redhat.com>