Adding ipv6_autoconf to Network Formula

This should allow for automatically assigned IPv6 network addresses
via SLAAC.

network/defaults.yaml

@@ -34,6 +34,7 @@ Debian:
       - ipv6ipaddr
       - ipv6netmask
       - ipv6gateway
+      - ipv6_autoconf
       - mtu 
     def_entries:
       - name: lo