Password-less SSH root access

So I had to configure password-less SSH access between a master machine and a slave one:

1. Create an SSH key pair on the master machine
root@master-machine$ ssh-keygen 

2. Create an SSH key pair on the slave machine,
root@slave-machine$ ssh-keygen

To copy the public key to the remote machine we need a root access, however by default password-based SSH access as root is not allowed

3. On the slave machine: sudo passwd.
3.1. set a password for root (if not already set)
3.2. edit /etc/ssh/sshd_config (not /etc/ssh/ssh_config) to change PermitRootLogin without-password to PermitRootLogin yes.
3.3. restart SSH deamon with service ssh restart, if in an ssh session service ssh reload.

4. Copy master's root public key to the authorized keys in the slave machine
root@master-machine$ ssh-copy-id -i root@slave-machine

Disable password-based SSH access for root:
5. On the slave machine, edit /etc/ssh/sshd_config to change PermitRootLogin yes to PermitRootLogin without-password.

6. Now you can ssh as root from the master to the slave machine without password:
root@master-machine$ ssh root@slave-machine

For more details on SSH keys, check link.