Fixes #3855 - Add specific package_methods rudder_yum to manage RPM on RHEL with less requests to RHN and use them on Techniques 'System', 'Inventory' and 'NTP'

Fixes #3855 - Add specific package_methods rudder_yum to manage RPM on RHEL with less requests to RHN and use them on Techniques 'System', 'Inventory' and 'NTP'

cf http://www.rudder-project.org/redmine/issues/3855