Sync with latest version of openstack.common.cfg

Sync with latest version of openstack.common.cfg

Cherry picks c1bf456 from Nova.

Changes since last sync:

 - Alphabetize imports in openstack/common/cfg.py
 - make reset() clear defaults and overrides
 - automatically create option groups
 - allow options to be marked as required
 - use a list comprehension instead of map()

Change-Id: Ie17fe855a75b5021b031902aa86031d8ddc06dfd
Reviewed-on: https://review.openstack.org/9867
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: James E. Blair <corvus@inaugust.com>