CDK-192: Allow dataset locations to be passed to the FS repo.

CSV imports need to store metadata using the FS repository, but pass in
the location of existing data. This removes the check that ensured
location was not set. This used to throw an exception, but is now
allowed.
CDK-192: Fix csv-import to run in parallel.

This uses a temporary repository to store the DatasetDescriptor for the
incoming CSV data, stored in the same FileSystem as the data. This is
compatible with the kite-data-mapreduce formats.