When connecting using SSH, check if host keys are available in the userdata

[moul]

Depends on: scaleway/image-tools#36

If the host keys are available you can :

• display fingerprints
• register fingerprint on the local host (ssh-keygen) + remove this: https://github.com/scaleway/scaleway-cli/blob/master/pkg/utils/utils.go#L83

I suggest you to begin with 1 as the userdata API is still very young and most of the actual images won't have scripts that generates the fingerprints