cloud-init: nocloud image support

cloud-init: nocloud image support

With configdrives we end up with the /etc/network/interfaces
file containing the interface names we use on the disk, ie.
eth0/eth1/..., which doesn't work on systems which do not
use this name.

With the 'nocloud' image type we can provide a
network-config in yaml which matches mac addresses. Ideally
we'd use version 2, but debian stretch ships with a too old
cloud-init for this, so for now we're writing version 1.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>