trying to find the same solution, maybe you have to rebuild rancher os image doing the following:
#cloud-config
ssh_authorized_keys:
- ssh-rsa YOUR_HASH rancher
hostname: gil
write_files:
- path: /etc/ssh/sshd_config
owner: root
permissions: '0600'
content: |
PermitRootLogin yes
PasswordAuthentication yes
This worked perfectly well.