- 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