cloudinit.net refactor: find_fallback_nic

[Dan Watkins]

This bug is tracking part of the refactor of cloudinit.net into
cloudinit.distros.networking, specifically the refactoring of the function
named in the title of this bug. See [0] for further details.

[0] https://cloudinit.readthedocs.io/en/latest/topics/hacking.html#cloudinit-net-cloudinit-distros-networking-hierarchy

[Ryan Harper (raharper)]

It would be good to refactor this to match what's in cloudinit/net/init.py:find_fallback_nic_linux() where we're looking for connected devices...

> carrier = read_sys_net_int(interface, 'carrier')
> if carrier:
> connected.append(interface)
> continue
> # check if nic is dormant or down, as this may make a nick appear to
> # not have a carrier even though it could acquire one when brought
> # online by dhclient
> dormant = read_sys_net_int(interface, 'dormant')
> if dormant:
> possibly_connected.append(interface)
> continue
> operstate = read_sys_net_safe(interface, 'operstate')
> if operstate in ['dormant', 'down', 'lowerlayerdown', 'unknown']:
> possibly_connected.append(interface)
> continue

Original discussion here:
  https://github.com/canonical/cloud-init/pull/453/files#r444272062

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3722