Add functional test for l3-agent metadata proxy

This change add a functional test for l3-agent metadata proxy to
verify:

 * iptables redirection from 169.254.169.254:80
 * headers added by the neutron-ns-metadata-proxy
 * proxy to the metadata_socket by the neutron-ns-metadata-proxy

Change-Id: I16f1269644a56d932426daa3a89dd5a8f747b314
Merge "Add functional test for l3-agent metadata proxy"