[GKE versioning] Update GKE version used to run e2e tests
[GKE versioning] Update GKE version used to run e2e tests

Right now the cluster versions provided are specific. But GKE allows
us to provide a version which follows versioning like `1.8` or `1.9`.

> 1.X: picks the highest valid patch+gke.N patch in the 1.X version

Above is the line copied from the GKE documentation:
https://cloud.google.com/kubernetes-engine/versioning-and-upgrades

So this commit changes the versioning to `1.8`, `1.9`, `1.10`.

Signed-off-by: Suraj Deshmukh <0abd0df55eb02d47bf2d3e301d3310c4d084e10b@kinvolk.io>

kinvolk/habitat-operator,kinvolk/habitat-operator