use user's primary group as root_group when user is not root

Hi,

In some circumstance, user's name is not identical to its primary group. When running not by root, concat will raise the following error,

Error: /Stage[main]/Concat::Setup/File[/Users/jianingy/.puppet/var/concat]: Could not evaluate: Could not find group jianingy

(my user name was "jianingy" but my primary group's name was "staff")

This patch fix it by adding another fact "concat_usergroup" which returns the name of user's primary group and set the default value of root_group to it.

Please have a look

Thanks very much