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.
If the options for running virtual display are set, then handling the ImportError exception will not have a different effect because the browser will not launch and will raise another exception. Raising an import error will be clear to the user that have missed something.