ifdown: Introduce _UnconfigureNIC() helper method

ifdown: Introduce _UnconfigureNIC() helper method

This method takes a NIC object, creates the proper invironment
and invokes ifdown script.

It takes an extra boolean argument only_local which is passed
to ifdown script as the second positional argument. It states
if the external script should wipe out every configuration
related to the NIC or should only clean up local thinks, e.g.
during a successful migration we are going to invoke ifdown
on the source node but we do not want to purge any DDNS
configuration related to instance's NICs.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>