Comment 2 for bug 1332231

This error is due to check with vnc_addr_mgmt.py to disallow subnet update.

                if ((req_subnet['enable_dhcp'] != db_subnet['enable_dhcp']) or
                    (req_subnet['gw'] != db_subnet['gw']) or
                    (set(req_subnet.get('dns_nameservers') or []) !=
                     set(db_subnet.get('dns_nameservers') or []))):
                    raise AddrMgmtSubnetInvalid(vn_fq_name_str, key)

Numan Siddique is working on fix to remove these checks (leaving the gw check behind)