traffic_replay: Assign users to groups by default

traffic_replay: Assign users to groups by default

The traffic_replay script has a myriad of options, but by default when
it creates user accounts it does not assign these users to any groups
(you have to specify extra options to do that). This isn't really a fair
test of samba performance, because it's unlikely that real world setups
will have users that are in no groups (other than the default ones).

This patch changes the default behaviour so that it will assign the new
users to groups automatically, if no other group options were
specified.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>