src: add tunnel template support

src: add tunnel template support

This patch adds tunnel template support, this allows to attach a
metadata template that provides the configuration for the tunnel driver.

Example of generic tunnel configuration:

 table netdev x {
        tunnel y {
                id 10
                ip saddr 192.168.2.10
                ip daddr 192.168.2.11
                sport 10
                dport 20
                ttl 10
        }
 }

This still requires the tunnel statement to attach this metadata
template, this comes in a follow up patch.

Joint work with Fernando.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>