Add option to run Selenium tests headless

[Maria Nita]

Selenium tests by default need a display and a browser driver for setup.
If no display settings are provided then the native display is used.
But if you run the tests on Jenkins or on a Vagrant machine, you usually want to run them headless.

In the setUp method of the BaseTestCase class, we define the Selenium webdriver. Tests inherit from this class.
Also we have a Selenium TestCase SeleniumTestCase, where we should also check for the option.

We can set an environment constant named WITH_DISPLAY to check, if we should start a headless session or not.
The default value should be True, which means that we run the tests with head = display.
If is False we can use https://pypi.python.org/pypi/xvfbwrapper/0.1.2 to start a headless session.

We should close the headless session on tearDown, if one was opened.

TODOs:

- In run_tests.sh we should add a new option to set the display type: head or headless. This will initialize a new environment constant - SELENIUM_WITH_DISPLAY
- Check for Selenium setup in TestCases and add a new condition to check the SELENIUM_WITH_DISPLAY value
- if the constant is set to True (default) then we will initialize the setup as it is now
- elif the constant is set to False, then we will start a new xvfb session (headless)
- at tearDown close the headless session if one is opened
- update requirements.txt with the new package

[Julie Pichon (jpichon)]

I believe this will help us get rid of the selenium-specific script in infra (see https://github.com/openstack-infra/config/blob/master/modules/jenkins/files/slave_scripts/run-selenium.sh), which means hopefully we could use regular tox jobs and avoid the problems around having a hard-coded command in run-selenium.sh (e.g. the integration tests also depend on selenium but run with a different flag, so can't use that script).

The review is currently open at https://review.openstack.org/#/c/79764/ .

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/79764
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=44055c167655166c8a2b44c28235c50734310d04
Submitter: Jenkins
Branch: master

commit 44055c167655166c8a2b44c28235c50734310d04
Author: marianitadn <email address hidden>
Date: Tue Mar 11 18:51:10 2014 +0200

    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