manifests/key.pp: make $secret mandatory

If $secret is not set the ceph-authtool call will produce and invalid
and simply empty keyring file.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
manifests/key.pp: add documentation for ceph::key

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>