allow the user to set a different value for PMX_VM_PRIVATE_IP

Solve the issue described in https://github.com/CenturyLinkLabs/panamax-coreos/issues/52 by allowing the user to set a different value for PMX_VM_PRIVATE_IP and using the current value as the default.

You may want to do something similar with the other env vars you are using. I made the minimum amount of change to solve the issue I was having.