Fixes internal server error for add/remove tenant flavor access request

Fixes the internal server error (500) response when an add or remove tenant access
to a flavor is made without specifying the tenant. Note that only the V2 API code
change is required because the V3 API version is protected by the input validation.

Change-Id: I7c126920016c7004fa55efb89747e97838d62f4e
Closes-Bug: 1253014
Merge "Fixes internal server error for add/remove tenant flavor access request"