Include licensing support for datastores in trove

We would like to handle license installation for Trove by creating a separate top level resource: "licenses". A license shall be associated with a single datastore during creation. After creation, a license can be passed in as an argument to a create or cluster-create command, which will be passed to the underlying guestagent for installation.

Licenses should support POST/PUT/DELETE, and a license associated with an instance or cluster should be changeable via PUT/PATCH.