Implement cloud-init integration

[chrisdoherty4]

Write a cloud-init data source adapter for the Hegel Metadata API.

[displague]

Related conversation in https://github.com/tinkerbell/hegel/issues/44

One limitation that we can expect with a Hegel data source adapter is that cloud-init will not be able to detect the cloud environment as a Hegel environment based on machine properties (typically DMI in the case of virtual machine cloud providers). The cloud-init team has not been inclined to enroll new providers with network-based environment detectors.

It is likely that a ds=hegel kernel parameter (or similar) will be needed by clients.