switchover.py: Add new options --replicating-master & --read-only-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). If a master is an intermediate or
passive master, --read-only-master must be used, so the switchover
can happen, but read_only is kept as 1 on all servers during all
Aside from the new procedures, new verifications were added to
check that both options have expected initial and ending state.