Backport 'ident' from python 3.3 for Oslo's SysLogHandler

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.

DocImpact
Closes-bug: #904307

Change-Id: I3815e6dc47e206e15f9da90ebbba5ca00c3ae641
Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>
Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>