vivian doesn't support ssh private keys with passphrases

vivian's wait_for_ssh (and other functions that use ssh) use -o BatchMode=yes, which disabled prompting for key passphrases.

The obvious fix for this is to not set BatchMode if a private key is specified (e.g. with make SSH_ID_KEY=foo vivian), but I'm not sure if that will be a problem later (cc @mupuf)