Use the preferred logging_user_identity_format option to specify an alternative context format
in log files. The removal of defining logging_context_format_string brings this more inline
with optimal production configuration of using the defined default. logging_user_identity_format
is set in devstack to maintain current compatibility of names instead of ids.
Change-Id: I3807d76b91b6cc1614b4a9a203509f8e1ad7146d