add function to dump cloudinit config

add function to dump cloudinit config

This adds a function to dump the generated cloudinit config. Only one
can be dumped at a time, either 'user', 'network' or 'meta'.

The logic to get user, network and metadata is copied from the other
path that also creates the ISO image to keep it simple and not
complicate the other code path further.

The hash generation for the metadata config is unified between nocloud
and configdrive2 formats. We need it a 3rd time with the new dump
functions so it makes sense to combine it and the metadata config
generation in a single function. The <format>_gen_metadata functions are
each used twice now.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>