Allow different ways of setting datastore secrets

[containerpope]

Description

At the moment secrets are written in plain text into the datastore config yaml. This leads to problems in deployment, because the config should then be deployed as k8s secret for example because it contains sensitive data.

Proposals:

Separate option to provide files containing the secret information. These files could then be mounted as secrets into the pod.

[containerpope]

@dcseifert what do you think ?

[dcseifert]

@mkjoerg With my latest commit for this issue, the datastore.yml-file can be expanded via environment variables. Therefore Secrets could be injected via mounting a k8s secret as environment variable

Does that also satisfy our needs, or is additional externalization into a file needed?

[dcseifert]

Closing this issue after a discussion with #56 as optional further enhancement.