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.
You can use `==` and `!=` for a partial match. `=~` is for regular expressions and this is not really one...

```shell
      if [[ "${output}" != *"kubernetes/pause"* ]]; then
```

Should work just fine... I'd invert the logic and handle the `==` case first and the other one in the `else` clause.