Allow priority definition for galera monitor nodes

There are situations where it would be beneficial to influence the node that the galeramon defines as "master" node in a galera cluster.

The current implementation that uses wsrep_local_state to define the master does not allow e.g. to define close nodes as master to make sure a majority of requests goes to them.

There is a priority_galeramon branch in the maxscale git at

https://github.com/mariadb-corporation/MaxScale/tree/priority_galeramon

I've compiled the above branch locally and it works as expected - it allows me to define a priority for each galera node and the one with the lowest priority is used as a master node by galera.

According to Markus Mäkelä (who is the author of the above branch) the branch was only meant as a prototype and he asked me to open this task to discuss merging the code.

Having this branch merged into a future release would be very much appreciated.