Can't re-import machine that already exists

[jakebesworth]

Trying to import a mydocker.zip when mydocker already exists does not overwrite it, when it should, or at least have a flag to allow overwrite.

Valid user story:

If you destroy a docker instance, with say Digital Ocean, and regenerate it with the same name, you would want to overwrite the credentials and information stored in ~/.docker/machine/machines/

Currently you have to rm -rf ~/.docker/machine/machines/mydocker when having a overwrite flag would be very helpful