Feature/3637 use tls wrapper mode

This changes the SMTP relay connection to the provider from STARTTLS to direct use of SSL connection. This change must be accompanied by:

*  A change in the platform to advertise port 465 on `smtp-service.json`.
* The merge of the following pull request: https://github.com/isislovecruft/python-gnupg/pull/28

(The pull request in python-gnupg has to do with a bug which i just perceived now that I fixed the SMTP relay tests.)