[cli] Add --nowebui arg to disable WebUI.

Allow the functional testing framework to properly test OWTF CLI without having
OWTF to freeze (See: https://github.com/DePierre/owtf_testing/issues/2 and #354)

+ Implement dedicated CLI server that does not start the WebServer, nor the
FileServer.
+ WorkerManager can be forced to exit when no job is remaining.