dashboard: allow configuring multiple grafana host

dashboard: allow configuring multiple grafana host

When using multiple grafana hosts then we push set the grafana and
prometheus URL and push the dashboard layout to a single node.

grafana_server_addrs is the list of all grafana nodes and used during
the ceph-dashboard role (on mgr/mon nodes).
grafana_server_addr is the current grafana node used during the
ceph-grafana and ceph-prometheus role (on grafana-server nodes).

We don't have the grafana_server_addr fact duplication code between
external vs collocated nodes.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1784011
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>