Move getEnvDefault to public API

Change-Id: I7a6fe88c4173e7e03c05d7deee305b4497b3eb77
Reviewed-on: http://gerrit.aldebaran.lan/46311
Reviewed-by: jlamotte <jlamotte@aldebaran.com>
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Approved: hcuche <hcuche@aldebaran-robotics.com>
Reviewed-by: hcuche <hcuche@aldebaran-robotics.com>
Tested-by: hcuche <hcuche@aldebaran-robotics.com>