Legacy's `Machine#id=()` has an important side effect when a nil ID is
specified - it clears the contents of the machine's DataDir.
We mirrored this behavior over to gogo, with a subtle difference - we
deleted the whole DataDir vs just its children.
It turns out the Docker provider relies on the DataDir being
cleared-but-not-removed by doing a SetId dance in its InitState action.
(see 1e6259dd00d702f83048c75c5c229ce6494c4c6e).
So here we need to mirror that behavior in order for the Docker provider
to work properly.
625806f448