tests: BaseException.message is deprecated, use str() instead

    IntegrationTests/src/bkr/inttest/server/test_model.py:743: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
      self.assert_('Invalid date format' in e.message, e.message)

Change-Id: I931f6b222c24fa47a9beb209789d61a47287597c