Use debian-9 GCE image family. (#7816)
Use debian-9 GCE image family. (#7816)

The debian-8 GCE image is deprecated so is not usable by default in the
deployment manager script.  So we'll bump up to debian-9 and that should
keep for a couple of years.
istio/istio,istio/istio,smawson/istio,geeknoid/istio,smawson/istio,smawson/istio,istio/istio,geeknoid/istio,istio/istio,geeknoid/istio,geeknoid/istio,geeknoid/istio,istio/istio,smawson/istio,smawson/istio,geeknoid/istio,smawson/istio