Integration test for rolling update

Rolling update is one of the most important things, but it's also one of the things people can make a lot of mistakes. These mistakes can block updating user cluster to the latest version of Druid, and so we should be able to catch them before it happens.

Integration tests for rolling update would be good for this purpose. Like our current integration tests, we can set up some docker images for some old releases and perform rolling update from them to the latest Druid built from master or PR branches. This integration test should be able to test rolling update while some tasks are running.

Start by locating the current integration tests and their Docker image setup. Review how those tests launch Druid versions, then define coverage for rolling updates from old releases to a current master or PR build while tasks are running. Done means the integration test detects rolling-update failures in those scenarios.

Written by the indexing model from the issue text.