Add some v2 flavor_manage API tests

On most APIs, the unit test coverage of v3 is better than the one of v2.
This patch ports v3 flavor_manage API tests to v2 tests for improving v2
API tests.

Change-Id: Ic52b68ea864ba9c4649fe22a431e5e55e72141c5
Merge "Add some v2 flavor_manage API tests"