feat: add SSH host keys role

• Added SSH host keys role with tasks to ensure /tmp/ssh_keys directory, download host keys from S3, and unpack them to /etc/ssh.
• Configured ssh_host_keys_s3_bucket and ssh_host_keys_s3_object variables in group_vars/all to support S3 bucket and object information for SSH host keys.
• Added { name: 'ssh_host_keys', tags: 'ssh_host_keys' } entry in cheaha.yml to define the SSH host keys role.