Couchbase sync gateway

Resolves jira task BROOKLYN-9
Instead use `entity.getAttribute(CouchbaseSyncGateway.ADMIN_REST_API_PORT)`.
If `ConfigToAttributes.apply(entity)` has previously been called (which it will have because this extends `SoftwareProcess`) then the attribute will have been set with the first free port in the range. However, it will be null if none of those were available.