As part of Secure RBAC work, support for system users(eg. system admin)
is being added. However current default format does not include
the system scope information so we can't determine whether the context
is non-scoped or system-scoped.
This change updates the default logging_user_identity_format so that
system scope is logged by default.
Change-Id: I7982cdb280599fb38362eb837cb527592267b91f