kubemci remove-clusters assumes that ordering of target tags has not changed

[nikhiljindal]

From #146:
// This assumes that the ordering of target tags has not changed on these instances.
// A potential solution to that problem is to recompute the target tags for the existing clusters,
// rather than removing the ones for old clusters.
// But we do not want to change existing tags for clusters that are already working.
// Ideally network tags should only be appended to, so this should not be a problem.
// TODO(nikhiljindal): Fix this if it becomes a problem.

Filing this issue to track the problem and see if users run into this.
cc @G-Harmon