Make Clusters a top-level entity in the data model

[markgoddard]

Change the clusters field in the TrustZone message to reference clusters by name or an ID, rather than embedding a list of Cluster messages in the TrustZone message. Clusters will become a new top-level entity in the data model, and will require data source methods to interact with them.

The limitation of one cluster per trust zone remains, and clusters will not be exposed in the CLI.

See #76 for details of the data source changes.