Load iptables rules from systemd unit file, not from nether program

Load iptables rules from systemd unit file, not from nether program

Drop nether support for loading iptables rules. Such rules should ideally be
managed from a central place that implements multiple requirements, not only
nether. It is not right for nether to be the manager of iptables policy.

For now nether daemon will stop managing the rules, they will be loaded from
systemd unit files. It is already done for ip6tables rules, as nether never
handled ip6tables for IPv6, only iptables for IPv4.

Change-Id: Icb0cf1f42b54e0859c182a6a4baac42e85294388