Allow MariaDB Monitor route read_query to the slave(s) of external server if needed

There is a proprietary request for the maxscale monitor "router_options" to label any server with [Slave of External Server] -> [Slave] so it can be used for read queries.

This is for topologies where MaxScale does not see the real primary and is only configured with the replicas. Currently, the state for any replica that appears to be successfully replicating is Slave of External Server, Running which is not a true status bit from the router's point of view. The desires feature request is to mark these sorts of servers as Slave, Running instead.

             ┌─────────┐              

        ┌───►│ Primary │◄───┐         

        │    └─────────┘    │         

        │                   │         

┌───────┼───────────────────┼────────┐

│   ┌───┴─────┐        ┌────┴────┐   │

│   │ Replica │        │ Replica │   │

│   └─────────┘        └─────────┘   │

│       ▲                   ▲        │

│       │    ┌──────────┐   │        │

│       └────┤ MaxScale ├───┘        │

│            └──────────┘            │

└────────────────────────────────────┘