Update Vagrantfile from Ubuntu 12.04 to 14.04

We have changed the osbash default from Ubuntu 12.04 LTS to
Ubuntu 14.04 LTS.

It's time for the Vagrant scripts to do the same.

Change-Id: Ide4180544aded732ca22e971e944dbe2f0717209
Merge "Update Vagrantfile from Ubuntu 12.04 to 14.04"