Sync log.py from oslo

This change syncs log.py from oslo-incubator with commit
79e8a9a08daf563aa8a8d9280c9a6a27dcafc8f2

To honor RFC5424 add use_syslog_rfc_format config option
(default False, would be deprecated in J after existing
syslog format deprecation) which adds APP-NAME to syslog message
before MSG part to reflect application or service name.
Usable only with use_syslog, otherwise ignored.

During J, the default logging format for syslog should be changed
to always provide APP-NAME, thus use_syslog_rfc_format could be
deprecated in J as well.

Since the rootwrap code was graduating to a separate library,
so the rootwrap related change should just go into oslo.rootwrap
and when oslo.rootwrap is released, nova will pull in the library.

Change-Id: I6018c7322314a4b57052be1ec0fc5fa437069f44
Closes-bug: 904307
Merge "Sync log.py from oslo"