This array structure is meant to be passed into the addDatabase
method and provides a suitable database cluster for each
environment of the specified site. When adding a database, a new
database will be added to each environment, and if an attempt is
made to add a database to an environment on a cluster that is not
associated with that environment the addDatabase call will create
a task that will fail.
Getting the array from this method guarantees that each
environment will have a suitable cluster_id. Any desired
modifications can be made and then the database add should work
correctly.