Consider improving `with_mount_unit` semantics

[jlebon]

Currently, with_mount_unit creates a mount unit with the What set to the same device provided to the filesystem block that uses it. But the device name used there may not be stable across updates (e.g. /dev/sda).

One thing we could do is have the generated mount unit use the label defined on the filesystem instead (i.e. /dev/disk/by-label/$LABEL). If the label is not set, and we detect the device name as "unstable", we can fallback to the device name but emit a warning.

See also: coreos/fedora-coreos-tracker#1732