Have a configuration parameter to automatically generate self-signed key materials

Follow up of https://www.mail-archive.com/server-dev@james.apache.org/msg70783.html

For security concerns, we should remove all
cryptographic keys from default configuration, including demo images.

We could then have auto-generation
configuration option to ensure both convenient and secure set-up for
demo image - we likely should consider implementing this too.

That way one would not need to choose between safety and (demo) conveniance.