traffic_replay: fetch domain from creds other than opts

traffic_replay: fetch domain from creds other than opts

For traffic_replay script, when user provides `--workgroup` or `-W` option,
it will be set on the creds option group, not the default opts one.

The previous code will not work properly when smb.conf file is missing.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>