platforms: add initial hetzner cloud support

[dedene]

Along with:

• providers: add initial hetzner cloud support ignition#667
• providers: add support for hetzner cloud afterburn#125
• coreos-base/oem-hcloud: add hetzner cloud support coreos-overlay#3490
• image_to_vm: add initial hetzner cloud support scripts#859

This will greatly enhance working with CoreOS on the Hetzner Cloud.

[dedene]

@LKaemmerling Would you mind taking a look at this from the Hetzner Cloud team? Feel free to suggest changes.

[LKaemmerling]

Hetzner Cloud does not have Private IPs (or VPN)

[dedene]

No but the metadata service seem to have an endpoint for it (although returning an empty string). So my thought was to already add it for when or if Hetzner Cloud would add private networking in some hypothetical point in the future. Would you rather prefer to remove this?

[ajeddeloh]

LGTM except maybe rename hcloud to hetznercloud.

[dedene]

@ajeddeloh Thanks for the feedback. See coreos/ignition#667 (comment) about my reasoning behind hcloud. Would you like me to change it to hetznercloud or can it remain hcloud?

[ajeddeloh]

As per the Ignition PR discussion, hcloud is fine. I'll hold off on merging this until the Ignition PR is merged.