set vm box to be hashicorp hosted trusty

I accidently left in the old download URL for coreos
even after I switched the box name to be ubuntu, it
worked fine for me because I already had that box
locally so it didn't try to download.  This should
resolve this by using the official bento/ubuntu-14.04
box which is a nice minimal image.

We also allow the default behaviour of sharing folder to VMs

By doing this we can stage our download files in a shared location
and speed up subsequent runs significantly.

Finally will create more verbose documentation.