Currently, when a new instance is allocated it randomly chooses one open AlmanacClusterDatabaseServiceType service and one open AlmanacClusterRepositoryServiceType service.

In the general case, this means that an instance could end up on dbx002 but on repox009. This is technically OK, but it's nice for now to have all the dbx004 instances be on repox004 since this is conceptually easier to reason about and react to if manual intervention is required to respond to an operational issue.

Implementing this would let us have multiple service tiers open simultaneously without losing the convenience of node correspondence.