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
Yeah -- there was a long thread/bug about that.  Vagrant doesn't have DNS and using /etc/hosts doesn't work as it doesn't get propagated into the pod/container.  And the env variable for the proxy uses this name so it needs to be resolvable from within the container.

Simplest thing was to revert back to using IPs as the identifiers.