Update to latest supported GKE version (#4759)
Update to latest supported GKE version (#4759)

Automatic merge from submit-queue.

Update to latest supported GKE version

According to https://cloud.google.com/kubernetes-engine/release-notes, the current version used is no longer supported; here we update to the next available 1.9 version.

Related to https://github.com/istio/issues/issues/262

/cc @ayj
geeknoid/istio,geeknoid/istio,istio/istio,smawson/istio,frankbu/istio,frankbu/istio,istio/istio,geeknoid/istio,smawson/istio,frankbu/istio,istio/istio,smawson/istio,frankbu/istio,geeknoid/istio,smawson/istio,frankbu/istio,smawson/istio,istio/istio,frankbu/istio,istio/istio,geeknoid/istio,smawson/istio,geeknoid/istio