DatasetRepositories.open should take a Configuration

The Loader for FS (and others) currently gets a Configuration using new Configuration() to set default values in registered URI patterns. However, this config is also used when instantiating FileSystem objects, which leaks its values that may have been overridden. We should instead allow the user to pass a Configuration object to open.

If users can pass Configuration objects to open, then it makes little sense to default URI settings from the environment because they will be available and more up-to-date in open. So we should also remove the environment config, set defaults to null, and detect when building the repositories what needs to be defaulted from the passed Configuration.

A work-around is to instantiate the FS before calling open and to use the cached instance.

Thanks to Ben Roling for pointing this out!