Command line option to list active config parameters

## Is your feature request related to a problem? Please describe.
Identified when documenting ocis config.

When reading the dev docs about the [configuration arithemtics](https://owncloud.dev/ocis/config/#in-depth-configuration), you get a clear warning that if you do not take very much care, a misconfiguration can sneak in resulting in a different behaviour than expected. This can happen because of the way a final configuration gets assembled (the configuration arithmetics) out of different yaml files. The issue is, that you have a bunch of yaml files, but currently you cant tell how the final assembled configuration looks like.

## Describe the solution you'd like
Have a command line option when calling ocis like `--print-config` which prints the final assembled and active configuration. It would be beneficial if such a print out is seperated by comments based on which extension it belongs.

## Describe alternatives you've considered
There is no alternative.

## Additional context
Points to be considered:
1. This is benificial when you want to review your final config AND
you can export the config for testing purposes
2. Any secret parameters need to stay secret - like we do in classic server when printing the config
3. Needs documentation in dev docs and in docs-ocis

Note, please file a docs-ocis issue when this gets implemented so we can update the documentation accordingly.

@butonic @wkloucek @dragonchaser (as discussed) @micbar @pmaier1
@EParzefall fyi