Inherit V2 keypair API's 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 between V2 and V3 Keypair APIs
Inheriting the V2 Keypair API's test from V3 tests.

Partially implements blueprint nova-api-test-inheritance

Change-Id: Ibe170a00e27619827678a1bbe104623ca24a81a3
Merge "Inherit V2 keypair API's tests from V3 tests"