validate: add checks for grafana-server group definition
validate: add checks for grafana-server group definition

this commit adds two checks:
- check that the `[grafana-server]` group is defined
- check that the `[grafana-server]` contains at least one node.

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible