Inherit V2 API's flavors tests from V3 tests

There is a lot of copy&paste test code for Nova v2/v3 API tests.
In addition, we need to add many checks for API test coverage.
As the result, we should apply the same changes to v2 and v3 tests.

This patch reuse the test code of flavors APIs between V2 and V3
Inheriting the V2 flavors API's test from V3 tests.

Partially implements blueprint nova-api-test-inheritance

Change-Id: I85463baec7af1e4e8bcc6310804fd15c1958d7ed
Merge "Inherit V2 API's flavors tests from V3 tests"