Galera cluster

[cvauvarin]

Add new module to monitor MySQL Galera Cluster.

Only the bare minimum is enabled by default :

• Check if the node is ready or not
• Check if the node is in the synced state (https://galeracluster.com/library/documentation/node-states.html#node-state-changes)
• Check the replication queue length (disabled by default as threshold can vary a lot depending on the cluster usage)
• Check the replication paused ratio (also disable by default for the same reasons)