FAILOVER and SWITCHOVER admin command to call external script

Calling external scripts on monitoring event is a good first step for failover.

It need additional feature to make the failover a 2 steps procedure:

• Many users would flavor a 2 step scenario
• Can be deliver to clients as default setup
• Removing ownership for failover tricky decision
• State = dead_master is subject to many false positive.

First reason to proceed like that is that switchover is most requested feature from users

• Potentially more frequently triggered when comparing it to hardware failure
• To upgrade hardware and software
• For rolling blocking database operations

Switchover is not control by monitoring but it's a user decision.

I would suggest that our first monitoring external script just send an email that :

• Warn the user to double check the reason the master is un-joinable
• Describe maxadmin command to failover
• List the current state of all backend
• Propose the best candidate for failover

This been done we need to map maxadmin cmd to external scripts

SWITCHOVER TO <backend_name>
FAILOVER TO <backend_name>

Following command pick the best candidate for the command
AUTO SWITCHOVER
AUTO FAILOVER

We need to add options in maxscale config file

for each backend
condidate_master=0 means the server can not be elected as potential master

General section
cmd_failover_slave_election_script=
cmd_failover_multi_master_script=

cmd_switchover_slave_election_script=
cmd_switchover_multi_master_script =