Add an option to disable password connections

Useful if one only wants to allow SSH keys. For now I use /dev/null as auth script, but it spams output (this is a classic scan) : 

```
DEBU[xxx] PasswordCallback: "guest" ""                 
WARN[xxx] Failed to execute password-auth-script: fork/exec /dev/null: permission denied 
DEBU[xxx] Server.Handle netConn=&{{xxx}}      
```
