Fixes bug #996190
Add framework to run selenium tests on the test slaves. This includes
installing the firefox and Xvfb packages, wrapping Horizon selenium
tests with a script that creates an Xvfb X server, and running the
Horizon selenium tests as check tests. The framework assumes selenium
tests will use the WebDriver API and not need any other pieces of
selenium (No selenium server etc).
Change-Id: I4f4cc502276890d617b5baaadf5f8a6eae1ba1ff
