Clean up the veth and ns created by the dchp agent

Override disable() in DhcpDriverNoOp, since we are not using
any external process for handling the dhcp and the superclass
method checks if a process is active. If it's not active
the clean up is not triggered

Delete the namespace in the interface driver unplug()

Signed-off-by: Rossella Sblendido <rossella@midokura.com>

Change-Id: I51f199b9714375c1753d5c616e60857e07542fb4