Share Certificates API's tests between V2 & V3

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 V2 and V3 Certificates APIs
Inheriting the V2 Certificates API's test from V3 tests.
Also removing the status code check which is different between
V2 & V3. Status code has been validated at client side.

Partially implements blueprint nova-api-test-inheritance

Change-Id: I98e03cecdbd36e521d007bbfb92a5f9818ff28f6
Merge "Share Certificates API's tests between V2 & V3"