Add CLI option for running Selenium tests headless

- Add requirement for xfvbwrapper
- Add CLI option for running tests headless
- Update Selenium TestCase to start virtual display if the option is set
- Update documentation about running Selenium tests

Change-Id: Icb1ac3491b8eef6c168bf1421cf073da67600982
Closes-Bug: #1290329
Merge "Add CLI option for running Selenium tests headless"