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.
Given the re-try logic, I think it's a little cleaner as is? I check to see if kubernetes/pause occurs in the list of containers -- any other result (error result from gcloud or otherwise) is considered an error which is retried up to a limit. So I prefer to keep this as is if you don't mind, thank you.