Regression test portability: system-specific cleanup

Add NetBSD system-specific cleanup code
Define killall as pkill when it is missing.

BUG: 764655
Change-Id: I3a1cfa3029fd60ede334a670fc115c211700063d
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8290
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Gluster Build System <jenkins@build.gluster.com>