Add start script & option for config template

[w-ensink]

We need the option to provide a config template to this issuer, as the config depends on secrets from different places and we want to be able to change those secrets without needing to redeploy the whole issuer.

This PR adds allows you to provide a config.json file in the form of a template and fill in the gaps using environment variables.

Using this mechanism only the pods need restarting, which can easily be done without redeploying.

Simple example

If you want to set a password based on an env var:

{
    "mail_password": "$MAIL_PASSWORD"
}

And then pass the MAIL_PASSWORD env var when starting the Docker container.

[jappe999]

Nice! Got one (possible) improvement.