Config-drive is an alternate no-network method for publishing local
instance metadata on OpenStack.  This change implements support for
fetching data from config-drive, and tries it before querying the
network metadata service (since config-drive will fail quickly if not
available).

Note config-drive involves mounting the filesystem with label
"config-2", so anyone using config-drive and running kubelet in a
container will need to ensure /dev/disk/by-label/config-2 is available
inside the container (read-only).