mon: add a MGR flag to mon commands

mon: add a MGR flag to mon commands

The flag will indicate that a command should be sent to ceph-mgr
instead of ceph-mon (rados_mgr_command instead of
rados_mon_command).

For legacy clients, we'll have to make the mon forward the message
on behalf of the client.

Signed-off-by: Sage Weil <sage@redhat.com>