Set default value for IPv6 subnet attribute flag

This patch would set the IPv6 subnet extended attributes flag
to True. The following thread has some additional information.
https://review.openstack.org/#/c/96723/

Change-Id: Ie6ac42646cc2a4d808c05126bf7dd51f2b03ee6b
Partially implements: bp tempest-ipv6-attributes-support
diff --git a/lib/tempest b/lib/tempest
index 79412f9..8681c45 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -64,6 +64,7 @@
 
 # Neutron/Network variables
 IPV6_ENABLED=$(trueorfalse True $IPV6_ENABLED)
+IPV6_SUBNET_ATTRIBUTES_ENABLED=$(trueorfalse True $IPV6_SUBNET_ATTRIBUTES_ENABLED)
 
 # Functions
 # ---------