Add support for providing preferred cluster

[Victoremepunto]

This allows a user to define a preferred cluster to override the default list of available clusters to test. It is useful to temporarily set a different set of server(s) for debug purposes.

# will use the `foo` and the `bar` cluster_ids as the preferred list cluster
AVAILABLE_CLUSTERS=("foo" "bar")

The list of configurable cluster ids must be of course a subset of the SUPPORTED_CLUSTERS list, any cluster id that is not supported will yield an error accordingly.