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'd prefer we move the KUBERNETES_NUM_MINIONS note to the a FAQ at the bottom.  If people just skim this they'll think that they should run this and be confused when things don't work.  This is optional and most users should be cool with 3 minions.