Python/ifconfig: T1680: fix dhclient shutdown
Python/ifconfig: T1680: fix dhclient shutdown

Tell DHCP server we release the IP address and also remove it from the interface
by properly calling dhclient shutdown.

Syslog:
Sep 23 21:20:34 vyos dhclient: Killed old client process
Sep 23 21:20:35 vyos dhclient: DHCPRELEASE on eth2 to 172.16.35.254 port 67

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x