GKE: Limit liveness.sh to server version >0.7.1

What I really want is
https://github.com/GoogleCloudPlatform/kubernetes/issues/2953, but
haven't had a chance to code that yet. Maybe it's time. (Then I'd
remove the provider-specific test and just say "is it > 0.7.1, or does
it claim to be capable of something from the future?" The latter
covers the HEAD server case .. though just bumping the server version
immediately after release might also accomplish that, too.)