Refactor class structure

The flavor tests don't need to be inherited from BaseV2ComputeTest since
_service already defines the service for the rest_client. It's fully
sufficient to use BaseTestCase.

Build a new calls called BaseNegativeAutoTest for all negative tests.

Change-Id: I6811ac1e085c9ae40425629b545429a61c1bbc39
Signed-off-by: Marc Koderer <m.koderer@telekom.de>