traffic_replay: Rework machine accounts to remove redundant code

traffic_replay: Rework machine accounts to remove redundant code

generate_users_and_groups() now generates the machine acounts as well as
the user accounts, so it seems there's no need to also have
generate_traffic_accounts(), which does the same job.

Instead, we can just pass through the number of machine acounts to
generate_users_and_groups() and delete the other function.

Also updated generate_users_and_groups() so that machine_accounts is
no longer optional (we want to create machine accounts in all cases).

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