Adds flavor support for Nova V3 API

The first part of adding flavor support for the Nova V3 API (support
for flavor-access API calls is still to come).

Restructures v1_1 testcases so as much as possible so they can be
reused for the V3 version of the tests. If it looks too ugly an
alternative would be to just cut and paste the fakes and test
from v1_1 to v3, which is simpler to understand but comes at the
cost of duplicated code. The upside though is it would be much
easier to remove v1_1/v2 support in the future.

Partially implements blueprint v3-api

Change-Id: Ic7cb3c43db02c07d37aea2675b310aaa50639c40