Add support for the IPv6 extended attributes in Tempest

Implements blueprint tempest-ipv6-attributes-support

Change-Id: I96bb78828eeb5548bb075f936342a027b9d11652
diff --git a/lib/tempest b/lib/tempest
index e899443..380c6c2 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -300,6 +300,7 @@
     iniset $TEMPEST_CONFIG network public_router_id "$public_router_id"
     iniset $TEMPEST_CONFIG network default_network "$FIXED_RANGE"
     iniset $TEMPEST_CONFIG network-feature-enabled ipv6 "$IPV6_ENABLED"
+    iniset $TEMPEST_CONFIG network-feature-enabled ipv6_subnet_attributes "$IPV6_SUBNET_ATTRIBUTES_ENABLED"
 
     # boto
     iniset $TEMPEST_CONFIG boto ec2_url "http://$SERVICE_HOST:8773/services/Cloud"