To my understanding the way to do this is to implement a specific run mode, i.e. a file clush.conf.d/root-ssh.conf

[mode:root-ssh]
ssh_user: root
ssh_options: -i <identitiy file to use> -oStrictHostKeyChecking=no

I see two problems with this

1. I always have to explicitly use --mode root-ssh
2. It is not possible to specify which nodes should use --mode root-ssh and which should use normal ssh in a single command.