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.
Here use `$(( attempt + 1 ))` (without the `$` and with more spacing) and quote the entire string:

```shell
  echo -n "Attempt $((attempt + 1)) to check Docker on node ${MINION_NAMES[$i]} ..."
```