Replace _private by _p

Change-Id: If93df708a4bc0a2525e63b91dd046def26b66940
Reviewed-on: http://gerrit.aldebaran.lan:8080/15841
Reviewed-by: ncornu <ncornu@aldebaran-robotics.com>
Reviewed-by: cgestes <gestes@aldebaran-robotics.com>
Tested-by: ncornu <ncornu@aldebaran-robotics.com>