Connecting REST API to DB, part II

In models.py (DB models)
* updated CLUSTER_STATUSES data structure to class for easier
  indexing and ability to add more extensions to statuses
* updated DB schema to make status a mandatory column

In api.py (DB api)
* added delete_cluster support
* added get_cluster support
* added get_clusters support

In v1.api (cue/api)
* completed support for get/list all clusters
* completed support for delete a cluster
* completed support for get a cluster

In test_models.py (DB models)
* Updated deleting tests to conform with new api

Change-Id: I3328556f482619dea9957827bfc44cac2b2373aa