selectCoordinators should be deterministic

[johscheuer]

Currently selectCoordinators in the https://github.com/FoundationDB/fdb-kubernetes-operator/blob/master/controllers/change_coordinators.go is not deterministic and returns different coordinators for multiple runs (even when the state is unchanged). That is not a problem currently since the operator only selects new coordinators when the old ones are not valid anymore. We should still change the code to always return a deterministic set of coordinators.