Add possibility to set node-red port from env variable.

Hey, all.

Lately I was trying to deploy node-red to [Cloud Foundry](http://www.cloudfoundry.org/index.html). Cloud Foundry sets ports for the web apps dynamically, to be able to work within CF apps need to be run on  port number that is equal to $PORT env variable. 

This quick fix will allow to run node-red into CF without additional changes.