Sync log.py from oslo

Sync log.py from oslo

This change syncs log.py from oslo-incubator with the
following commit:
* Backport 'ident' from python 3.3 for Oslo's SysLogHandler
  79e8a9a08daf563aa8a8d9280c9a6a27dcafc8f2
* default connectionpool to WARN log level:
  1978114030c275c571a1786644a5e0d51ada29c0
* Fix deprecated messages sent multiple times
  52b64466177b58c8e80bc72c5ce1e940782c969a
* Remove None for dict.get()
  86707cd5c01a2de6b9ba386d7ea4f72c9c7286b1
* Rename Openstack to OpenStack
  61ff7a66fef51ba2a783a5ed87e14256a6e9432c
* Use ContextFormatter for imparting context info
  dda47c9cef19c5d8f050cf5081535b4061024457

Change-Id: I14c3d7eccc596783e380ebd57110337d3ce49038
Closes-bug: 904307