Jenkins ssh-agent only takes user and private key as credentials for build jobs, not user and password, otherwise you will get error below:
FATAL: [ssh-agent] Could not find specified credentials
[ssh-agent] Looking for ssh-agent implementation...
[ssh-agent] Exec ssh-agent (binary ssh-agent on a remote machine)
.....
Permission denied, please try again.
Permission denied (publickey,password).
Target remote server must have completed setting for ssh login via private key before. (public key in ~/.ssh/authorized_keys)