feat(ssh_host_keys): add role for managing SSH host keys

- Ensure the `/tmp/ssh_keys` directory exists.
- Download SSH host keys from S3 and unpack them to `/etc/ssh`.
- Restart the SSH service to apply the new keys.
- Add necessary variables for S3 and AWS credentials in `groupvars/all`.
- Include `ssh_host_keys` role in `cluster.yml` playbook.