Restore "configuration blacklist" functionality

There used to be code to provide a "black list" of Spark configuration entries that users weren't allowed to set. That code was removed since a lot of the configurations it was trying to protect didn't really have security implications.

But that depends on running Spark in "yarn-client" mode. So, at the very list, Livy admins should be able to block users from choosing which Spark master they want to use.

Given that, restoring the black list itself is probably better, since it gives admins more control over how users can run things on their cluster.