bootstrap process for Xpand should be region-aware

As part of XPT-518 (Transylvania), Xpand will now support Multi-Region High Availability Clusters that:

• Allows Xpand to be deployed across multiple regions and zones
• Provides full, zero RPO high availability in the face of a regional failure

If regions are configured, there will be 3 regions (Primary, Secondary, and Observer) and it is expected that the customer will have MaxScale deployed in each region (perhaps optional for the Observer).

For the first iteration, Xpand supports sending traffic to the Primary region only. The secondary is a standby, in case of complete failure of the Primary region (and we expect that the user will have to handle failover, e.g. using Route 53 or the like).

The bootstrap process needs to be updated so that it is region-aware, and only sends traffic to the relevant region (e.g. MaxScale in the Primary should spread connections across nodes in the primary region only).