contint: Add Xvfb module, role::ci::slave::localbrowser and Chromium

* Implement Xvfb module (based on Xvfb module in mediawiki-vagrant by ori-l).
* Move general provision of browsers from contint::browsertests to contint::browsers.
* Use Xvfb and latest chromium-browser in contint::browsers.
* Use contint::browsers in role::ci::slave::browsertests.
* Add new role::ci::slave::localbrowser.

Change-Id: If2e96bfa9a1c46ab34af9526f0c3f241a3d17056