mgr: helper function to check if a service is a normal ceph service

mgr: helper function to check if a service is a normal ceph service

This would be widely required since ceph metadata server entries are
maintained in service map (DaemonServer::pending_service_map). Such
normal ceph services would need to filtered when processing the service
map to avoid extraneous entries getting processed.

Signed-off-by: Venky Shankar <vshankar@redhat.com>
(cherry picked from commit 79503fc16749ed0cfe8a89ea3b3c8c792d6b8809)