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.
This looks incorrectly indented... Can you please fix it?

I think in the block above (`gcloud` call in the `if`), you want to indent the continuation lines 4 spaces, then the block inside the `if` (starting with the `if (( attempt > 5 ))`) should be indented 2 spaces, that means 2 before the end of that line. That's similar to Python/C++ usual indentation. Does that make sense?