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'

Conflicts:
	techniques/systemSettings/misc/clockConfiguration/1.0/clockConfiguration.st
Merge pull request #380 from nperron/bug_3855/dev/add_new_package_methods_and_fix_package_method_for_rhel_rudder_2_6

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'
Merge branch 'branches/rudder/2.6' into branches/rudder/2.9