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
I'm not sure that we need to say anything here.  The only reason vagrant has special username/password is that we don't have a way to generate/pass a random password through.  Things should work by default with `~/.kubernetes_auth`.