traffic_replay: Move machine account creation

traffic_replay: Move machine account creation

I was assuming that generate_users_and_groups() only gets called in the
--generate-users-only case. However, it also gets called in the default
traffic replay case.

This patch reworks the code so that the number of machine accounts to
create gets passed in, and the 'create 25% more computers than users'
assumption only applies to the --generate-users-only case.

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