openstack: Add Nutanix AHV support

[olljanat]

Nutanix AHV uses cloud-init/Ignition which is based on OpenStack implemention.

However it provides only OpenStack metadata without EC2 metadata which why current logic added by #462 fails to error:

# journalctl -u coreos-metadata
Jan 10 06:39:45 localhost systemd[1]: Starting coreos-metadata.service - Flatcar Metadata Agent...
Jan 10 06:39:45 localhost coreos-metadata[1423]: Error: failed to run
Jan 10 06:39:45 localhost coreos-metadata[1423]: Caused by:
Jan 10 06:39:45 localhost coreos-metadata[1423]:     0: writing metadata attributes
Jan 10 06:39:45 localhost coreos-metadata[1423]:     1: failed to open file '"/tmp/afterburn-EZcXvr/ec2/latest/meta-data.json"'
Jan 10 06:39:45 localhost coreos-metadata[1423]:     2: No such file or directory (os error 2)
Jan 10 06:39:45 localhost systemd[1]: coreos-metadata.service: Main process exited, code=exited, status=1/FAILURE
Jan 10 06:39:45 localhost systemd[1]: coreos-metadata.service: Failed with result 'exit-code'.
Jan 10 06:39:45 localhost systemd[1]: Failed to start coreos-metadata.service - Flatcar Metadata Agent.

So this PR simply adds logic which allow OpenStack metadata to be returned even when EC2 metadata is not available.

Tested with this custom build in Flatcar Linux:

# journalctl -u coreos-metadata
Jan 10 10:35:22 localhost systemd[1]: Starting coreos-metadata.service - Flatcar Metadata Agent...
Jan 10 10:35:23 localhost systemd[1]: Finished coreos-metadata.service - Flatcar Metadata Agent.