Add support for arbitrator nodes

For MXS-6782, the arbitrator node needs to be indicated by some mechanism. Currently it can be added into servers and then excluded from promotion via servers_no_promotion but this is a workaround and not a neat solution.

Add support for a setting (e.g. arbitrators) which lists one or more servers which are only used with cooperative_monitoring_locks. This approach keeps the server list clean and allows services to use cluster to refer to the monitor without causing traffic to be sent to the arbitrator node.