Regression test portability: useradd/userdell

Adjust useradd/userdel flags for NetBSD. Similar work will have to be
done for other non Linux systems

BUG: 1129939
Change-Id: I2a30819a4915d04473f0ef0c0c063de6015212a1
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8895
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>