In order to be able to use new mounter library, this PR adds the
mounterPath flag to kubelet which passes the flag to the mount
interface. If flag is empty, mount uses default mount path.
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).