(Bug 46920) Add Puppet class for configuring qa/browsertests

This change adds a Puppet class that will clone the 'qa/browsertests'
repository and configure browser testing using cucumber and Selenium.

Currently experimental, and thus not enabled by default. To enable, you'll need
to uncomment the line "class { 'browsertests': }" in puppet/manifests/site.pp
and run "vagrant provision".

If everything worked, connect to your instance by running 'vagrant ssh -- -X'
(the extra arguments are required to enable X11 forwarding). Then run:

    $ cd /srv/browsertests
    $ bundle exec cucumber features/login.feature

FIXME:

 * MediaWiki is served on port 80 inside the VM and 8080 outside it, by dint of
   the port forwarding setup. Because the common way to access the wiki is from
   the host, $wgServer is set to "http://127.0.0.1:8080". MediaWiki uses this
   value to construct URLs to static assets, which the tests are consequently
   unable to pull. Should be simple to fix, but I haven't decided on the best
   way to do it yet.
 * The 'mediawiki::user' resource type sucks. It should accept an 'ensure'
   parameter and actually check if the user exists using some sane method. I
   have not identified a good existing maintenance script that is usable for
   this purpose.
 * The module docs need to be improved.

Change-Id: I18825437e1f0ad005b1e4f6db850968603573b01