Throttle browser tests at 1 per node max

Might help with browsertests timing out with xrvb/SauceLabs.

Require the Jenkins plugin 'Throttle Concurrent Builds'
https://wiki.jenkins-ci.org/display/JENKINS/Throttle+Concurrent+Builds+Plugin

Change-Id: I370554615608e9332be43ca267c05fc744cbeb0a