Update config properties to be a defaultdict

All asked keys for the config will return None if the properties file is
not available or the key is not present. This will make docs build but
will need to think better on the framework configuration in the future.
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.
Merge pull request #903 from Ichimonji10/update-config

Update config