No KeyError when the family does not exist

If the user has a username in user-config.py for a family
which does not exist, pywikibot will not load, failing in config2.py
with a KeyError.

This change silently accepts unknown values for the three dicts
which are pre-populated with family names:
- usernames
- sysopnames
- disambiguation_comment

Change-Id: I614096aa2396f8a0794225da4ffad891f20bf6ab