Add option to run a window manager

Allow run a window manager for UI tests running using virtual display
feature. If the window manager command specified in the properties file
is found then it will be run before open any browser window. This will
allow the browser window be maximized.
How about:

`If you want to run your tests in a maximized window, you could set the ``window_manager_command`` attribute in the configuration file to whatever window manager you have installed. For example, assuming that you have a light window manager such as `Fluxbox` installed, then you could set window_manager_command=fluxbox.`