Ref #3002 : Add rollback feature to Rudder

Linked to http://www.rudder-project.org/redmine/issues/3002

ticket 3102 needs to be accepted before (make commit before archive) http://www.rudder-project.org/redmine/issues/3102) pull request : https://github.com/Normation/rudder/pull/58

so when 3102 will be accepted the first commit will disappear. 

It fixes http://www.rudder-project.org/redmine/issues/3029 (backend) and  http://www.rudder-project.org/redmine/issues/3030 (UI)