mgr: request mon metadata when receiving a report msg from an unknown monitor

mgr: request mon metadata when receiving a report msg from an unknown monitor

* Mgr needs to issue an metadata request when receiving a report message
  from a monitor who hasn't been in the mgr's daemon_state. Otherwise the
  connection between mgr and monitor cannot be established.

Fixes: http://tracker.ceph.com/issues/37753
Signed-off-by: Xinying Song <songxinying@sensetime.com>