Allow Tez local mode to run against filesystems other than local FS

In TEZ-717, I incorrect thought setting fs.defaultFS programmatically in tez-site would work for local mode.
Currently the requirement is that tez-site.xml must have fs.defaultFS set to file:///.

While that works, it doesn't allow for seamless execution in either local-mode or on a cluster.

The main issue here is that when Inputs / Outputs are configured - they use a version of configuration which reads tez-site, and do not use the configuration from the client itself (which is correct behaviour).

Not sure what a good way to fix this is
1) It may be possible to override this value each time an instance of Configuration/TezConfiguration is created. One possible way would be to statically add a default resource to Configuration the moment a local client is created.
2) Provide information in the contexts on whether this is local or not. This is fairly ugly, and would get in the way of running mixed mode tasks.

Anyone have other suggestions ?