usbprinter: Add new group

Add a new usbprinter group with cups and saned as users. This allows
CUPS and lorgnette to access USB printers owned by this group.

BUG=b:158035459
TEST=Verify usbprinter group exists on DUT

Cq-Depend: chromium:2231480
Change-Id: I535142e398613358b1f608543a21d1f691289896
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2231089
Tested-by: Jesse Schettler <jschettler@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Jesse Schettler <jschettler@chromium.org>