Descriptors with file: schema URLs don't work with Hive/Impala

I just created a test dataset (with the CDK-139 branch) with a file:/... schema. The schema URL was set in Hive rather than the literal and Hive couldn't read the schema to query it. The short-term fix is to check for hdfs: URLs or use the literal. Eventually, we'll probably want to create a HDFS URL, and this starts overlapping our recent discussion on a schema manager.