Support for the creation of self-signed certificates

[bgrieder]

The Certify operation of #101 allows the creation of certificates from public keys but only supports signature by another key.

Nothing in the KMIP specification describes how this could be achieved. The proposed solution is to pass a well-known value, say [SELF SIGNED] to both the PrivateKeyLink and CertificateLink of the Attributes in the Certify request.

[bgrieder]

Will tackle this one after the LUKS PR is passed