debirf: Create persistent shared host key for SSH

[frzb]

For SSH host keys are an essential securtiy feature against man-in-the-middle attacks.
On each start of a Coinboot node a new host key is generated by the OpenSSH daemon and needs to be acknowledged when initiating a SSH connection or is ignore by the SSH client configuration at all.

In a controlled cluster environment where access is only happening in the local network with a minimal risk for man-in-the-middle attacks sharing host keys is acceptable. So we have to:

Find a way to create a persistent shared host key.