Add network-down script

Add network-down script

Analogously to lxc.network.script.up, add the ability to register a down
script. It is called before the guest network is finally destroyed,
allowing to clean up resources that are not reset/destroyed
automatically. Parameters of the down script are identical to the up
script except for the execution context "down".

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>