cloud-init should parse initramfs rendered netplan if present

This bug was originally filed in Launchpad as [LP: #1861460](https://bugs.launchpad.net/cloud-init/+bug/1861460)

Launchpad details

affected_projects = ['ubuntu-z-systems', 'casper (Ubuntu)']

assignee = raharper

assignee_name = Ryan Harper

date_closed = None

date_created = 2020-01-30T23:04:46.020620+00:00

date_fix_committed = None

date_fix_released = None

id = 1861460

importance = wishlist

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1861460

milestone = None

owner = raharper

owner_name = Ryan Harper

private = False

status = in_progress

submitter = raharper

submitter_name = Ryan Harper

tags = ['installer', 's390x']

duplicates = []

_Launchpad user **Ryan Harper(raharper)** wrote on 2020-01-30T23:04:46.020620+00:00_

initramfs-tools used to only execute klibc based networking with some
resolvconf hooks.

In recent releases, it has been greatly improved to use
isc-dhcp-client instead of klibc, support vlan= key (like in
dracut-network), bring up Z devices using chzdev, and generate netplan
yaml from all of the above.

Above improvements were driven in part by Oracle Cloud and in part by
Subiquity netbooting on Z.

Thus these days, instead of trying to reparse klibc files in
/run/net-*, cloud-init should simply import /run/netplan/$device.yaml
files as the ip=* provided networking information on the command line.
I do not currently see cloud-init doing that in e.g.
/cloudinit/net/cmdline.py