Refactor QA configuration code

Ever since its introduction (sometime before commit cec9845c in September
2007), the QA configuration was stored in a dictionary at module-level
in “qa/qa_config.py”. The configuration was loaded, verified and
evaluated using module-level functions. Since then the configuration has
become more complicated and more functionality has been added. This
patch refactors handling the configuration to use a class and provides
unittests.

- The configuration is loaded through a class method which also verifies
  it for consistency
- Wrapper methods are provided in “qa_config” to not change the
  interface
- Unit tests are provided for the new configuration class
- The configuration object is still stored in a module-level variable
  and can be retrieved using “GetConfig” (direct access should be
  avoided so an uninitialized configuration can be detected)

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>