Modifications and improvements to the vagrant cluster README

This adds more details about using the cluster once it's up.
It also corrects the credential file being used, which is hard coded by kubecfg.sh
And it clarifies the use of KUBERNETES_NUM_MINIONS which can be set to 1 if you don't want replicas.

Thanks for the consideration

-sebastien
Agreed. I removed that last line. We could remove the entire section about auth, since it gets hard coded by kubecfg.sh. I kept it there because that was in the original.