Currently we allow tenant access for public flavor, however,
we can't query it after that because flavor is public.
This patch adds check for add access function to raise exception
if the flavor is public.
Also, a nit change is use methods introduced in
793bcc07b9 to get flavor.
APIImpact: Adds new 2.7 API microversion due to new error condition
in flavor access API
Implements blueprint check-flavor-type-before-add-tenant
Closes-Bug: #1361476
Change-Id: I461175e9969a0dd5b2b7ef75ea7d9f36f3a306d0
6abb88befe