Make it easy to turn on MRM behaviour

No modifications are needed in order to cause MaxScale to launch the script used for performing failover when the master goes down.

All that is needed is to add something like

[Database Monitor]

type=monitor

module=mysqlmon

script=/bin/mariadb_failover ...

events=master_down

...

to the the monitor configuration. However, since the script is a MariaDB product, it could and should be much end user friendlier, for instance, like:

[Database Monitor]

type=monitor

module=mysqlmon

master_promotion=on

...
Internally that would still be handled as the launching of an external program.