rancid: also create system group for system user

since I027c780ac5f6fe we use the standard puppet type
to create the user, which means now we also have to create
the primary group for it

currently on netmon1001:

root@netmon1001:~# id rancid
uid=997(rancid) gid=1001(rancid) groups=1001(rancid),1004(systemusers)

Change-Id: Icb0784519093fbceeb55e68d9e589cc3715b0522
Merge "rancid: also create system group for system user" into production