Extract etcd membership validation into a separate phase

[kke]

Fixes #768

In conjuction with #769 makes it possible to do something like:

applyOpts := &action.ApplyOpts{....}
applyAction := action.NewApply(applyOpts)
validateEtcdPhase := &phase.ValidateEtcdMembers{}
applyAction.Phases.Remove(validateEtcdPhase.Title())
err := applyAction.Run()

When using k0sctl as a library, this will make apply action skip the etcd member list validation.

[kke]

Would this resolve the issue for your use-case, @tppolkow?

[tppolkow]

@kke Thanks for looking at it! Ideal situation for us would be to not have to modify any k0sctl code and perhaps skip this via a flag. If that is not an option then I think this would work as a second best option, it is certainly better than the workaround we currently have which is much more involved.