[system-tests] Improve controllers scaling test to cover dynamic nodes removal as well

[Bogdan Dobrelya]

Curent "Check HA mode on scalability" test group is:

        Scenario:
            1. Create cluster
            2. Add 1 controller node
            3. Deploy the cluster
            4. Add 2 controller nodes
            5. Deploy changes
            6. Run network verification
            7. Add 2 controller nodes
            8. Deploy changes
            9. Run network verification
            10. Run OSTF

I suggest to update it as the following:
Check HA mode on scalability

        Scenario:
            1. Create cluster
            2. Add 1 controller node
            3. Deploy the cluster
            4. Add 2 controller nodes
            5. Deploy changes
            6. Run network verification
            7. Add 2 controller nodes, 1 compute node
            8. Deploy changes
            9. Run network verification
            10. Run OSTF ha, sanity, smoke
            11. Delete the primary and the last added controllers.
            12. Deploy changes
            13. Run OSTF ha, sanity, smoke

[Aleksandr Didenko (adidenko)]

11. Delete the primary and the last added controllers.

After that we will have 2 controllers and problem with quorum. Are you sure it's the test case we want to cover? Maybe remove primary and one of the last added controllers to have 3 controllers in total with quorum?

[Egor Kotko (ykotko)]

https://review.openstack.org/#/c/198643/