Add `--provision` option

[amotl]

Like vagrant up --provision, we might want to add this flag to the racker run command line in order to run re-provisioning of the WDM Vagrant machine.

When already having this flag, it can also be used to reset the checkout of the Windows Docker Machine repository.

https://github.com/cicerops/racker/blob/719ce6187ee44606e6ffa329d13d47353fc41b56/postroj/winrunner.py#L51-L53