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
Looking over this, we actually can support multiple replicas with a single minion as long as you do not externally bind the port.  The vagrant setup today does per pod ip properly, so maybe we want to just update examples to create N pods via replica, and then bind a service to the pod on a specified port.