Create open_perms context manager for custom or multiple writes.
Create generate_config for a small removal of redundancy.
0640 root:adm the resulting files.
When user configures network with subiquity, it's rendered
netplan should be wholly definitive. So, we remove the other files
that may have config. This fixes a bug where running in an instance
when running on a system where cloud-init had rendered a 'match' with
'macaddress'.
When writing netplan we keep 'macaddress' match in place but drop
others. The others may just wildcard from the installer environment,
but macaddress are likely by cloud-init or otherwise intentionally
written.
Also add an atomic write in subiquitycore/file_util and move the
netplan code into subiquitycore/netplan.py, and add some unit test
helpers from cloud-init.
Dan Bungert