UCLOUDNG-1495 Patches for DHCP infinite lease time

With these fix, it is possible to set infinite lease by setting "dhcp_lease_duration" in /etc/quantum/quantum.conf to "-1". (default of this config is 120s and we're using default currently)
But before apply infinite lease time to any deployment, it would be better to solve "https://bugs.launchpad.net/neutron/+bug/1271344" issue first.