That would explain it.
~/root/.ssh/authorized_keys does not contain your SSH key. If you’ve followed this guide for password-less SSH http://dev.hortonworks.com.s3.amazonaws.com/HDPDocuments/AMBARI_DOC_SUITE/index.html#Item2.5 you would end up with two files:
1. rsa_id
2. rsa_id.pub
rsa_id is your SSH key and rsa_id.pub is the one you put into the autorized_keys files, which could contain many of this entries.
Use rsa_id as your key for Ambari install.