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 the v2
API.

Closes-bug: #1212179

(cherry picked from commit 4054cc4a22a1fea997dec76afb5646fd6c6ea6b9)

Conflicts:
	nova/api/openstack/compute/plugins/v3/servers.py
	nova/api/openstack/compute/servers.py
	nova/tests/api/openstack/compute/plugins/v3/test_servers.py
	nova/tests/api/openstack/compute/test_servers.py

Change-Id: I681ae9965e19767df22fa74c3315e4e03a459d3b