Vagrant: install edx-platform under /opt

Compatibility with the Filesystem Hierarchy Standard: http://www.pathname.com/fhs/
Merge remote-tracking branch 'singingwolfboy/vagrant' into vagrant
vagrant: Moves from `/opt/edx-platform` to `/opt/edx/edx-platform`

To allow the vagrant user to create the /opt/edx/[log|data|db] folders