False positive for '.format' check

[Stephen Finucane]

Hacking 4.0.0 modifies the H904 that ensures delayed interpolation of strings to catch instances using '.format' like the following:

  LOG.info("Test {}".format(foo))

Unfortunately this is picking up false positive in nova, due to the presence of a 'format_message' method on the base exception it uses [1]. This means examples of the following now fail:

  LOG.warning(e.format_message())

Personally, I think it seems like that are many objects with a 'format' method that we might want to use in logging, meaning this check is overly broad and probably not fit for purpose. I'm open to ideas on how to minimize false positives though.