Expose output parameters matching root XML node, fix GetBucketLocation

Python's xml.etree.ElementTree find operation doesn't perform matches on
the root node, so that case is now explicitly covered. This is done as a
fallback to minimize impact on existing code.

The S3 API call GetBucketLocation returns a naked LocationConstraint,
which would previously get lost during XmlResponse processing. The
fallback now added in XmlResponse.find makes the output parameter
available to clients.