Resolve "Remove ES PVCs without breaking gke-uninstall"

The problem why it was not working on our first try was because we were using a post-sync hook with many pods still running and associated with that PVC. Now we use the cleanup Helmfile hook which occours after post-sycn. Moreso, we give 30 secs for the Pods to be all terminated before we delete the PVCs.

Closes #29 (closed)