Group Creation and Update - Vagrant system-wide installation

The group are not created and updated due to multiple reference to a ressource that should be unique.
Explain in this closed bug : http://tickets.opscode.com/browse/CHEF-3474

Also, change the vagrant recipe as it is more simple to install chef as a gem on system install instead of using original chef install on a fresh install due to the fact that the ruby system could be different as the new ruby causing link issue.