Give Vagrant VM a static IP address

This sort of sucks, because we can't guarantee that 10.10.10.10 (or whatever) won't clobber something on your local network, but I'm not sure what a better solution is.

Ideally we'd also use something like this to set up a `docker` hostname or similar: 

https://github.com/smdahlen/vagrant-hostmanager

Unfortunately installing Vagrant plugins is slow, which makes this complicated. I'll open a new issue about that.