expand test suite framework to run on top of docker and vagrant in addition to virter

[mmguero]

Currently the test suite runs on top of virter. This is good, and it works well, but we could potentially expand it to work with other backends as well.

this would involve a few things

• changing the main unit to take arguments allowing you to specify the backend to use
• use object oriented programming to refactor the MalcolmVM object so that there are different derived classes that encapsulate the backend logic

Ideally we could maybe reuse some of the provisioning steps to be usable across the various types, although there may be some duplication that has to occur.