mgr/cephadm: fixing Grafana domain handling

mgr/cephadm: fixing Grafana domain handling

it seems that with Grafana 10.4.0 the domain parameter is taken into
account while building the final url (earlier versions didn't seem to
behave the same way). This change sets the domain to the hostname where
Grafana daemon is running instead of '*.lab'. serve_from_sub_path is
removed as it's no needed and when add it causes some undesirable
redirections that could break monitoring HA.

Signed-off-by: Redouane Kachach <rkachach@ibm.com>