Forbid SSH password fallback

[Bencaddyro]

When you clever ssh if your key is not properly configured (ssh not finding it, key not registered on cc side etc...) it will try to fallback by asking a password. However on clever cloud side ssh session can only work with ssh key so it is bound to fail.

Can we try to modify the ssh configuration so that password authentication for this session is forbidden ?

[mehdi653]

Opened a fix for this in #1120: clever ssh now passes PreferredAuthentications=publickey and PasswordAuthentication=no so a missing or unregistered key fails immediately instead of prompting for a password that cannot work.

E2E tested against a real Clever Cloud app (valid key connects; unregistered key exits without a password prompt).