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.
Not sure I understand why the `--project` argument is being dropped here, it seems to be used elsewhere... I haven't really tested this myself now but I have 2 or 3 projects and I'm often not using the default one, so I'm not sure that by removing this argument you're not breaking it... Can you expand on the rationale of removing this parameter?