Tests that use run_coro are at risk of being broken by the introduction
of another test using IsolatedAsyncioTestCase. Switch over to only use
IsolatedAsyncioTestCase.
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