Add missing operations for k8s command

[nvthongswansea]

Add missing operations for k8s command such as: create, update, delete, etc.

[JanLukasGithub]

Why are there gscloud-kubernetes and gscloud-kubernetes-cluster, and to which should these new operations belong to?
To me gscloud-kubernetes-cluster looks like an unnecessary abstraction, as almost all kubernetes commands operate on clusters, and the others don't take any argument and are easy to understand even without a difference between gscloud-kubernetes and gscloud-kubernetes-cluster