[SPARK-1395] Allow "local:" URIs to work on Yarn.

This only works for the three paths defined in the environment
(SPARK_JAR, SPARK_YARN_APP_JAR and SPARK_LOG4J_CONF).

Tested by running SparkPi with local: and file: URIs against Yarn cluster (no "upload" shows up in logs in the local case).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #303 from vanzin/yarn-local and squashes the following commits:

82219c1 [Marcelo Vanzin] [SPARK-1395] Allow "local:" URIs to work on Yarn.