Support RedHatEnterpriseWorkstation in add-rule

RedHatEnterpriseWorkstation is a variant of RHEL 7, and a possible
output from "lsb_release -si", so add-rule needs to support checking for
that value.

Change-Id: Ie57ba4e94a50129737018d63b121d4c0b1dcc6dc

Conflicts:
	elements/iptables/bin/add-rule