Scott, did you configure password-less ssh, for a root-access level account, from the Ambari-server?
The Ambari-based installation instructions discuss how to generate a public/private key pair and configure public-key-based authentication (i.e. password-less) for ssh using ssh-copy-id.
This would need to be enabled for the user account Ambari-server runs as; alternatively you can provide the credentials for root user accounts for all of your Ambari-agent nodes during cluster deployment.