Update Vagrant dev environment

 - Updated Ubuntu image to use 16.04.
 - VirtualBox provider to use centos/7 image + vagrant-vbguest plugin.
 - Improved docs so it is clear that vagrant-vbguest plugin is only
   required with VirtualBox.
 - Clarified comment and made it independent of the release.
 - Refactored the kolla and kolla-ansible paths to be based on the
   username used by Vagrant, that can change depending on the image.

Change-Id: I23d84dcc3297c43eb12eff9c443d282281fc6c82