Ensure dnsmasq accept rules are preset at startup.

Fix bug 1000853.

This bug reported that after restarting nova-network, the dnsmasq ACCEPT
iptables rules were no longer present, causing instances to lose their
IP addresses.  This patch updates the restart_dhcp() function in the
linux_net driver to ensure these rules are present even if dnsmasq was
already running.  Before this was only done when first starting dnsmasq.

(cherry picked from commit aa1e71d)

Change-Id: Icda3364d3a61018b912cea7a4c96b2cbcc1fbdd7

nova/network/linux_net.py

@@ -689,6 +689,7 @@ def restart_dhcp(context, dev, network_ref):
         if conffile.split("/")[-1] in out:
             try:
                 _execute('kill', '-HUP', pid, run_as_root=True)
+                _add_dnsmasq_accept_rules(dev)
                 return
             except Exception as exc:  # pylint: disable=W0703
                 LOG.error(_('Hupping dnsmasq threw %s'), exc)