Add Robot test to verify Container Balancer for RATIS containers

Currently there are only unit tests for Container Balancer and no acceptance tests at all. At a minimum, we should add a Robot test to verify Container Balancer for RATIS containers. And probably in the future we should add robot test for EC case.

Test case:
1. Move 1 datanode to maintenance mode (we use 4 datanodes in this test)
2. Create multiple keys  (after loading the data, we check that 3 datanodes are ~60% busy, and the one that is in maintenance mode is empty)
4. Start datanode recommission (wait until datanode recommissioning is completed)
5. Start container balancer (wait until container balancer is completed)
6. Check results (after balancing on all 4 datanodes, we should see approximately the same data distribution.)