Use 201 instead of 204 for flavor creation success code

Per https://github.com/openstack/api-wg/blob/master/guidelines/http.rst#success-codes,
we should be using 201 Created when a new flavor is created. As this status will
require a response body, we should consider emitting the JSON representation of
the newly-create flavor.

Change-Id: Ia43651ada0a239f7e3d76cdd2e379a08fd35fd4e
Merge "Use 201 instead of 204 for flavor creation success code"