Fixes #3931 - Change rudder-agent cron file to use /opt/rudder/bin/check-rudder-agent

Fixes #3931 - Change rudder-agent cron file to use /opt/rudder/bin/check-rudder-agent

See http://www.rudder-project.org/redmine/issues/3930 and http://www.rudder-project.org/redmine/issues/3931

Need to be merged after https://github.com/Normation/rudder-packages/pull/119