[SPARK-1522] : YARN ClientBase throws a NPE if there is no YARN Application CP

The current implementation of ClientBase.getDefaultYarnApplicationClasspath inspects
the MRJobConfig class for the field DEFAULT_YARN_APPLICATION_CLASSPATH when it should
be really looking into YarnConfiguration. If the Application Configuration has no
yarn.application.classpath defined a NPE exception will be thrown.

Additional Changes include:
* Test Suite for ClientBase added

[ticket: SPARK-1522] : https://issues.apache.org/jira/browse/SPARK-1522

Author      : bernardo.gomezpalacio@gmail.com
Testing     : SPARK_HADOOP_VERSION=2.3.0 SPARK_YARN=true ./sbt/sbt test
@tgravescs tested for Hadoop 0.23 and 2.4.0. A bit silly that I made such mistake when the main issue was actually accessing through reflection such fields within the `ClientBase` classs.