Treat RHEL 7 and later like Fedora w/r/t iptables

RHEL 7 replaces legacy init with systemd, and must be treated like
Fedora w/r/t iptables persistence.  This patches checks
operatingsystemrelease in addition to operatingsystem in order
to differentiate between RHEL 7 and earlier releases.

Both RHEL 7 and recent Fedora releases require the iptables-services
package to support the commands in lib/puppet/util/firewall.rb.

This also corrects the path to /usr/libexec/iptables/iptables.init.