Enforce flavor access during instance boot

The code in the servers API did not pass the context when retrieving
flavor details.  That means it would use an admin context instead,
bypassing all flavor access control checks.

This patch includes the fix, and the corresponding unit test, for both
the v2 and v3 APIs.

Closes-bug: #1212179

Change-Id: I681ae9965e19767df22fa74c3315e4e03a459d3b
Merge "Enforce flavor access during instance boot"