boa: boa.conf: run under the nogroup group

The default boa.conf we install specifies that boa should run under the
nobody group, but we don't have such a group in our default skeleton (and
boa doesn't add it), causing boa to fail to start:

[01/Jan/1970:00:00:10 +0000] No such group: nobody

Instead use the nogroup group, which is presumably what was meant.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>