The ifupdown commands (ifup and ifdown) are not useful on many modern
distributions. Instead, the "ip link set" set of commands are standard
linux utilities and should be available everywhere (see
http://baturin.org/docs/iproute2/).

ipupdown utilities have the added quirk that they return 0 on failure,
which makes the changed line fail silently in case ifdown didn't bring
the network down.

After the interface was brought back up, renew DHCP requests to ensure
the network is actually useable.
Signed-off-by: Chris Glass <chris.glass@canonical.com>