Make multiple attempts to sanity check (GCE) and validate (ALL) cluster

This PR changed the sanity check for GCE cluster creation to make multiple attempts to deal with slow cluster birth. It also performs a more detailed check of the Docker installation (by ensuring kubernetes/pause can be found). It also adjusts validate-cluster.sh (for all providers) to make multiple attempts to validate a cluster.

Before, even when creating a modest sized cluster (say, 12 nodes), these steps failed not because there was anything wrong with the cluster but just because these steps can be slow and we don't retry.
Hmmm... I'd suggest quoting the string, but that will in effect change the behavior because if you quote it instead of expanding to user's home it will print a literal `~`, though I'm not sure if that's not really a good thing...

If you really want the user's home, maybe you should be explicit with:

```shell
  echo "Wrote ${HOME}/.kubernetes_auth"
```