certificate: add XML signature to security element

certificate: add XML signature to security element

Certificate Provisioning Service expects a XML signature as part of the
WSSE Security element in the SOAP header. The digest is calculated from
the Timestamp element and that should be correct already.

I haven't figured out how to correctly generate the signature value
using the SignedInfo element, key data and HMAC-SHA1 digest algorithm.
I'm pretty sure what is missing is the correct key data...