misleading message for import-image call with unknown import-method

The function glance.api.v2.images.ImageController._validate_import_body checks the import method specified in the body of the request against the list of configured enabled_import_methods, and if it isn't found returns the message: "Unknown import method name '%s'." This is a bit misleading because, for example, if the operator has decided not to allow the glance-direct method, it's not really an unknown method, it's just not enabled at that site.

An improvement would be something like: "Import method '%s' is not supported at this site."

It might be overengineering to define a list of KNOWN_IMPORT_METHODS = ['glance-direct', 'web-download'] and then give the "not supported message" for those and the "unknown" message for any others (unless we already have a list like that somewhere that we're already maintaining).