Refactor module robottelo.common.__init__

Move method `Config.get_root_path` to function `_get_app_root`. The method
never referenced `self` and had no logical connection to the class, so there
was no reason to keep it in the class.

Trim down method `Config.__init__` and move some of its functionality into
function `_config_file`. Use this method in place of some formerly hard-coded
debugging strings.

Thoroughly improve docstrings, use the `format` string method, and generally
clean up the module.