Add IPV6 API test cases for Neutron Subnet API

This bp:ipv6-subnet-attributes will add new API testcases for IPv6
subnet attributes (ipv6-ra-mode and ipv6-address-mode).

Change-Id: I4c37ba6a8968f31a030037e20abeecf1b970f862
Merge "Add IPV6 API test cases for Neutron Subnet API"