Galeramon: Server-side initiated maintenance mode

Hi,

It would be useful to do (sort of) graceful failover of a galera node from the server's side. This is useful in case we want to do scheduled maintenance on the machine, for example automated install of os security packages. It is complicated to login to each maxscale instance so it would be very useful to handle this from just the MariaDB instance that is taken down.

Please add some features in galeramon:
When a server has wsrep_desync (config) enabled, remove the 'Master' and 'Slave' flags. Or maybe even the 'Synced' flag but I don't know what side effects that has.
Also nice to have: When a galera node gets the 'read_only' flag, the 'Master' flag should be removed.

Thank you,
Michaël