--virtualbox-import-boot2docker-vm could default to boot2docker-vm

[bfirsh]

Could this:

$ docker-machine create -d virtualbox --virtualbox-import-boot2docker-vm boot2docker-vm dev

just be this?

$ docker-machine create -d virtualbox --virtualbox-import-boot2docker-vm dev

[nathanleclaire]

I don't think it's very common to have your boot2docker VM named anything else, so I was in favor of doing it this way (BoolFlag), but it is true that it could be named something else. Maybe we could default the flag to boot2docker-vm and check if is has changed to determine whether or not to proceed with a migration. cc @ehazlett