Communicate to users how to pre-emptively change the location of the vm files

[glinton]

This comment shows that docker-machine can create a machine and store the vm files at an alternative location (currently defaults to the user's home directory).

Maybe we can communicate to users that by setting the MACHINE_STORAGE_PATH environment variable prior to the first nanobox run, nanobox(docker-machine) will place vm files where they desire.

[glinton]

We'll also need to communicate how to set the location of docker native files (images/containers)

[notxarb]

I wonder if we could/should have this as a nanobox config setting, and then set that environment variable when running nanobox-machine/docker-machine commands