Support for more than one running VM, with a different VM_NAME

I have a need to spin up more than one VM to represent different production servers in a local dev environment.

b2d works nearly well enough, except that the virtual disk is shared between two running VMs.  By using VM_NAME as part of VM_DISK, two different VMs don't have to share one vmdk file.

This pull request should allow me to do something like the below and have it "just work"

    VM_NAME="frontend" DOCKER_PORT=4000 SSH_HOST_PORT=3000 boot2docker init
    VM_NAME="backend" DOCKER_PORT=4001 SSH_HOST_PORT=3001 boot2docker init