Allow to request metadata proxy only with redirection

Allow to request metadata proxy only with redirection

metadata service should be requested on 169.254.169.254:80 and router
namespace iptables rules redirect the request to the metadata-ns-proxy
on 127.0.0.1:$metadata_port. But currently the metadata-ns-proxy can be
requested directly on $router-ip:$metadata_port.

To avoid such behavior, this change marks packets redirection in mangle
table (PREROUTING), redirects (PREROUTING) them in nat table, accepts
them in filter table (INPUT) using the mark. Packets send to the
metadata proxy port without mark (so directly) are dropped. The
mark can be configured through the new option metadata_access_mark.

Remark: redirected packets are not local packets (in general), so
setting metadata proxy server host to 127.0.0.1 will disallow direct
queries but so redirected queries.

DocImpact
Partial-Bug: #1187102
Change-Id: I6a9bb12c8bf68c6fcf4e4060f8dfe44a309a41da