mon/MgrMonitor: read cmd descs if empty on update_from_paxos()

mon/MgrMonitor: read cmd descs if empty on update_from_paxos()

If the MgrMonitor's `command_descs` is empty, the monitor will not send
the mgr commands to clients on `get_descriptions`. This, in turn, has
the clients sending the commands to the monitors, which will have no
idea how to handle them.

Therefore, make sure to read the `command_descs` from disk if the vector
is empty.

Fixes: http://tracker.ceph.com/issues/21300
Signed-off-by: Joao Eduardo Luis <joao@suse.de>
(cherry picked from commit 3d06079bae0fbc096d6c3639807d9be3597e841a)