switchover.py: Enable new option --replicating-master
--replicating-master allows a master switchover to happen on even
if the master is replicating originally, and it stops and resets
its replication; and after switchover happens, it sets it back on
the newly promoted master.
This option, disabled by default, allows a switchover to happen on
intermediate masters or circular replicating masters, automating
the disabing and reenabling of replication.
Note that it still requires the master to be a real master (read
only option must be disabled).