Regression test portability: sha1sum

Provide a shell function for sha1sum if missing

BUG: 764655
Change-Id: I8b232af8aab7f95ba7c7ad83e37a111d562201a5
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8450
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>