Ability to export certificates from Stork

Stork stores its certificates in a database. @tomek feels (and @fdupont agrees) that there needs to be an ability to import and export certificates. Here are couple usecases:

1. there is a problem with TLS and it needs to be investigated. The standard practice is to inspect the certificates using openssl.
2. admin wants to inspect the traffic and decode the traffic, e.g. wireshark allows such ability, but it of course requires providing the necessary secrets.
3. an audit wants to inspect certificates and perform some form of automated checks

A more advanced case would be this:

4. a deployment with high security requirements would want to generate its own certs and keys and provision them to Stork. This by definition would be a manual process

Since the last item requires import capabilities, it is currently out of scope for this ticket. But it would very useful and also the next logical step after we get the export capability.