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
Merge "Set default value for IPv6 subnet attribute flag"