Fix hardcoded gateway in IPV6 subnet api tests

This patch removes the hardcoding and calculates the Gateway-ip
based on the tenant_network_v6_cidr value.

Change-Id: Iee4c450714caf49d6753d9455e6ab3698b64340d
Closes-Bug: #1318962
Merge "Fix hardcoded gateway in IPV6 subnet api tests"