Afterburn v4.0.0

Changes:

• rename project from coreos-metadata to Afterburn
• introduce a cl-legacy feature flag for backward compatibility with Container Linux
• change metadata attribute prefix from COREOS_ to AFTERBURN_ in non-legacy mode
• read ignition.platform.id kernel argument instead of coreos.oem.id in non-legacy mode
• drop merging of authorized_keys.d into authorized_keys in non-legacy mode
• providers: rename ec2 -> aws, gce -> gcp in non-legacy mode

Bugfixes:

• providers/gce: fix panic fetching metadata

Misc:

• providers/gce: add basic hostname mock-test
• rustfmt whole project