Made organize imports compatible with OpenStack standards

This adds three options that can be used to make 'organize imports'
compatible with the OpenStack hacking standards
(http://docs.openstack.org/developer/hacking/#imports), specifically
H301 and H306.  (Some of the other standards can already be applied
with rope, like H303, while others, like H302, are well beyond the
scope of this change.)

The three options are added to default_config.py, and documented
there.