Defer applying iptable changes when nova-network start

When restarting nova-network service, nova-network will initialize iptables and
remove old rules, then add floating ip iptable forward rules one by one.

If there are a lot of floating ips on one host, the process of adding floating
ip rules could last several minutes, which will cause those floating ips that
last added iptable rules disconnected for same time.

This patch defers applying nova-network iptable rule changes until
initialization is done instead of adding floating ip forward rules one by one.

Change-Id: I12651850b44790aef3817fedacae050bff57e364
Closes-bug: 1308390
Merge "Defer applying iptable changes when nova-network start"