The OpenStack cloud provider retrieves mounted Cinder volume paths
by looking in /dev/disk/by-id, expecting the disk serial IDs (e.g.
SCSI ID) to include the volume ID.
The issue is that not all hypervisors are able to expose this.
For example, Hyper-V will just preserve the original Cinder volume
lun SCSI ID (without setting the volume id). For this reason,
disk path lookups will fail.
In order to be able to leverage Hyper-V based OpenStack providers,
as a fallback, we're querying the metadata service, searching for
disk device metadata. Note that starting with Nova Queens, the Hyper-V
driver always provides disk address information through the instance
metadata.
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).
