Using CSW harvester OutputSchema is ignored while gmd is imposed

Hi,
CSW is a very flexible and powerful standard which allows clients to query a catalog in several ways.
Normally a getCapabilities request is performed by the client to understand which formats are available etc.
The OWLlib is a good library but it still lack lots of low level functionalities like this (https://github.com/geopython/OWSLib/issues/605).
I'm starting a discussion to introduce changes to that library but in the meanwhile I don't see why we can't have this here as well.
F.e. we could use the CSWharvester to gather metadata with different output schema formats at the moment **gmd** is imposed!!!

I'm going to propose a simple pull request (tested) which allows us to specify the target schema from the harvester configuration so **if the server supports that schema** it would be possible to obtain metadata in different formats.

My usecase is:
1. I'd like to query GeoNetwork with **csw** asking for iso19115* specifying the outputschema
2. I would like to specify it into the configuration (per harvester configuration).

With the PR I'm going to propose it would be possible.