* Add 'allow_create' field to Flavor DB model and the corresponding
  migration.
* Extend API for flavors to return the 'SNF:allow_create' attribute
* Update POST /servers API call to check that the flavor that is
  specified in the request has the 'allow_create' attribute set to True.
  Otherwise, Forbidden(403) is returned. Also, add a relevant unittest.
* Extend 'flavor-modify' and 'flavor-list' management commands to handle
  the 'allow_create' flag
* Update administrator guide and Changelog