Add log messages for virtual display feature

Sometimes the window manager process hangs and the test suite stuck. To
better find the cause was added logging messages to show when a virtual
display and a window manager is created, will be stopped and stopped.
The log messages will help debug the hanging problem and also will
provide more information about what is going on on.

Bonus: docstring formatting for robottelo.test module.
Force kill window manager if it hangs

Sometimes the window manager hangs and blocks the UI automation. Now the
window manager process will be force killed if it does not terminates in
30 seconds.

Also improve logging to show the window manager command as a string.
Kill window manager instead of trying to stop it

Fluxbox insists in not stop gracefully and the UI automation still
hanging. Because this, is better to send SIGKILL directly.
Merge pull request #1685 from elyezer/satellite-6.0.z.1

Cherry pick commits related to killing the window manager