Add well-known sorting when selecting targets

[maruina]

The target selection returns the list as we get that from the controller-runtime client. We should have a predictable order because

• easier to test
• easier to debug
• if we released bad code we make sure we always update the same clusters first, reducing MTBF

Note that we don't want the same order for every ProgressiveRollout CRD as we would end up having the same "canary" cluster for every service. Using the PR CRD name as seed might be a good idea

[maruina]

We have a predictable order because we sort clusters and app by name. We should probably find a better way to do that when we implement #10