Add uption to create (POST) metadata in update_metadata function in the resource class

This MR fixes the update_metadata function in the fact that there is no way to create metadata for existing file objects in Coscine that do not have metadata assigned yet (e.g. in RDS-S3 resources). The function doc string did state it handles this, but this was not the case.

Furthermore, I have also included an option to overwrite files or not in the upload function (may have been smart to do this in another MR... I could revert and split things, let me know).