gre/tun: init ip6gre

gre/files/tun.j2

 {%- for name, interface in pillar.ifaces.items() if 'mode' in interface -%}
-{%- if interface.mode == 'gretun' -%}
+{%- if interface.mode == 'gretun' or interface.mode == 'gretun6' -%}
 auto {{ name }}
 iface {{ name }} inet tunnel
+{%- if interface.mode == 'gretun' -%}
 	mode	gre
+{%- elif interface.mode == 'gretun6' -%}
+	mode	ip6gre
+{%- endif -%}
 	endpoint	{{ interface.remote }}
 	local	{{ interface.local }}
 	{% if 'ttl' in interface %}ttl	{{ interface.ttl }} {% endif %}