Separates tempest api tests to categories

To provide an ability of separate test running for tempest gate job we
create three test categories:
1. Stable - for tests that should always pass
2. In progress - for tests that fall because of bug
3. Not ready - for tests, that cover not implemented, but expected
functionality.

Implements: bp separate-tempest-tests
Change-Id: I6e3636ffc4f6505e206b2c319185f2a67fdd73f7
Merge "Separates tempest api tests to categories"