Disallow HCloud servers without SSH key

[janiskemper]

/kind feature

Describe the solution you'd like
No SSH key should be usually (otherwise please comment) a configuration issue. We can easily catch that and make users aware of this.

This will introduce a breaking change and will affect everyone provisioning HCloud servers without ssh key.

However, I think the drawbacks are not as severe and the advantage will be that no-one can misconfigure it.

We will add a condition to HCloudMachine that there is no ssh key.

The ssh key can be provided by the feature of adding it to the Hetzner secret even on the fly, without changing any templates.