mgr/cephadm: Add GrafanaSpec.initial_admin_password

mgr/cephadm: Add GrafanaSpec.initial_admin_password

By default, we're not creating any admin accout for Grafana now,
but we're adding an option to set the grafana password manually using:

```yaml
service_type: grafana
spec:
  initial_admin_password: mypassword
```

Users can then easily log into Grafana with the given password.

Fixes: https://tracker.ceph.com/issues/48291
Signed-off-by: Sebastian Wagner <sewagner@redhat.com>
(cherry picked from commit fdae665a2fe645d3f76b212f440ffe2c96281bc0)

Conflicts:
	src/pybind/mgr/cephadm/tests/test_services.py