Make GET /flavors response structure consistent

Return a JSON object instead of a JSON array as the response representation.
This is consistent with the response representation of GET /services. Further,
it allows for the possibilty of adding a top-level "links" property in the
future.

Change-Id: I64fabfb5974eb928d84f937b125d4826cc86b78a
Merge "Make GET /flavors response structure consistent"