xfstests: add local.config example

xfstests: add local.config example

I think it'd be useful to have a local.config template in the
git repo, to avoid having to re-read the README each time
to remember the required environment variable names.

Thoughts?  Am I just lazy?  :)

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>