Check metadata iptables chains during functional test

Check metadata iptables chains during functional test

The L3 agent functional test creates a router and asserts that
the proxy process is up. It will now also check that the NAT
and filter rules were added via the iptables manager.

This is to allow us to move the metadata management out of the
L3 agent and into its own L3 agent driver.

Change-Id: Iaa9b475a4294db96a9645829d362e090b61ed3a2
Partially-implements: blueprint restructure-l3-agent