Modify Vagrantfile so ch-grow and buildah work out of the box

[rstyd]

This is a follow on issue to #491 . We need to modify the vagrant file to include skopeo, umoci, and the other dependencies required for ch-grow.

[reidpr]

Let's have Docker as well as the two unprivileged builders.

Also, we'll want to test all three. Perhaps we should change to standard scope so we don't spend all day testing it (full scope would be 2 hours × 3).

[reidpr]

Also need to remove the privileged Buildah stuff.