Use Ubuntu's 'best mirror' for pulling from the repos

[dhruvsinghal]

The current VM provisioning pulls repo contents from the Ubuntu's US mirror, which is pretty slow outside North America (for example in India). Might be a good idea to use a base box that uses the mirror selector as described in: hashicorp/vagrant#1200

[craig-riecke]

Turns out this is tricky. If I simply use the patch (which overwrites the entire file /etc/apt/sources), then some required packages can't install. But you can't just add the mirrors onto the end of /etc/apt/sources either.

Still it's worth doing ... people say it speeds up package downloads even within North America. So I'll do some more checking...