Fix path handling for user files in Livy

Livy has some spotty file path handling in a few places.

The first one is the weird "absolute" vs. "relative" paths, which I'm removing in LIVY-70. The result after that is that users have to always provide an absolute path, which is fine.

But that means users should not provide paths to local files, because there's no (easy) way for them to exist on the Livy server. Instead, one of two options should be taken:

• disallow paths without a proper scheme (e.g. "http://..." is fine, "/foo" is not)
• set up a default URI (could be "fs.defaultFS" read from Hadoop's configuration?) that is prepended to scheme-less paths.

The second problem is in handling uploaded files for client sessions. Those are written to "livyHome", which is actually a local path, not an HDFS path. We should have a separate configuration for where to write these files on HDFS. On top of that, we need to write these files with the proper permissions, respecting the session owner and proxy user when applicable (related to LIVY-52).