The Nova SSH key distribution plays are subject to the global behavior of --limit in playbook invocation. This means if you follow the scaling documentation to add a single new compute host with `openstack-ansible setup-openstack.yml --limit localhost,NEW_HOST_NAME`, you'll end up with a new compute host that only knows its own SSH keys, and none of the pre-existing compute nodes will know the SSH key of the new one.
I assume there's no way to override the limit argument at the play level to force a given play to run on all or a specified set of hosts regardless of limit, although in this scenario that would seem like the best option.
Failing that, I think the scaling documentation needs to be updated to explicitly state the need to run the os_nova-install playbook with the nova-key tag whenever a new host is created; if you agree I'll be happy to open a merge request for such a change.
Same problem over here, the --limit option with Ansible is a real trap and should be avoided in most cases.