Use the IPv6 documentation prefix as the default cidr

Don't use a cidr which might be routable, instead use the
IPv6 documentation prefix which should be dropped and
filtered by routers according to RFC 3849.

Change-Id: I6f0ec34f7c2178a659d6119d2e33b09f135a8caf

tobiko.conf.example

@@ -108,7 +108,7 @@
 #ipv4_prefixlen = 24
 
 # The CIDR block to allocate IPv6 subnets from (string value)
-#ipv6_cidr = 2003::/48
+#ipv6_cidr = 2001:db8::/48
 
 # The mask bits for IPv6 subnets (integer value)
 #ipv6_prefixlen = 64

tobiko/openstack/neutron/config.py

@@ -28,7 +28,7 @@ OPTIONS = [
                default=24,
                help="The mask bits for IPv4 subnets"),
     cfg.StrOpt('ipv6_cidr',
-               default='2003::/48',
+               default='2001:db8::/48',
                help="The CIDR block to allocate IPv6 subnets from"),
     cfg.IntOpt('ipv6_prefixlen',
                default=64,