Add more e2e tests

[timebertt]

What would you like to be added:

More e2e tests covering the following:

• scale down of the controller (movements)
• scale up of the controller (drain + reassignments)
• graceful shard termination, shard lease cleanup
• failure detection: non-graceful shard termination
• basic e2e tests with the webhosting-operator

Why is this needed:

While working on the thesis, I didn't add any e2e tests for the sake of finishing the implementation part of the thesis.
#196 added some rudimentary e2e tests, but the test suite doesn't cover the entire set of mechanisms/scenarios.
Now that the project is supposed to mature and get more stable, having good test coverage is crucial.