Further refactor robottelo/common/__init__.py

Prevent class `Config` from returning a default value of `None` when its
instance var `properties` is read. Instead, make `properties` an normal dict.
If calling code reads a nonexistent value from `conf.properties`, (and this
will happen for all reads if no config file is found) that calling code now
receives a KeyError exception.

It may be useful to set some default values in the configuration object. For
example, a default locale may be appropriate.