contint: puppet class to setup browsertests slaves

We are going to run web browser tests against VisualEditor in labs
instances.  The new role::ci::slave::browsertests class provides:

* dependencies for the Jenkins slave agent. Currently that is just the
  openjdk-7-jre-headless package.  I have split it up from the
  role::ci::slave class which is production specific.
* A home dir on /mnt/jenkins-workspace to avoid GlusterFS and use
  /dev/vdb where all the disk space is.
* dependencies needed to install qa/browsertests.git which is a ruby
* application providing its own list of gems.
* a localhost vhost listening on 127.0.0.1:9413 and bound to the labs
  directory mnt/localhost-browsertests which has plenty of space.

Note that labs provides a jenkins-deploy user, so we do not have to set
up that user like we are doing in role::ci::slave.

bug: 54385
Change-Id: I35f3a9b738fc8f1c594674e63d21e545eca77000
Merge "contint: puppet class to setup browsertests slaves" into production