EXP-454. Allow express.py to specify JAR files to exclude from the classpath

 - Remove the call to TmpJarsTool.
 - Adds new user-classpath entry precendence config key.
 - Cleanup how java lib paths are computed.

issue: https://jira.kiji.org/browse/EXP-454
review: https://review.kiji.org/r/2080