yarn APPLICATION_CLASSPATH needs to be overridable

Right now MRApps sets the classpath to just being mrapp-generated-classpath, its content and a hardcoded list of directories.
If I understand correctly mrapp-generated-classpath is only there for testing and may change or disappear at any time

The list of hardcoded directories is defined in hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-api/src/main/java/org/apache/hadoop/yarn/api/ApplicationConstants.java at line 92.
For convenience, here is its current content:

  /**
   * Classpath for typical applications.
   */
  public static final String[] APPLICATION_CLASSPATH =
      new String[] {
        "$HADOOP_CONF_DIR",
        "$HADOOP_COMMON_HOME/share/hadoop/common/*",
        "$HADOOP_COMMON_HOME/share/hadoop/common/lib/*",
        "$HADOOP_HDFS_HOME/share/hadoop/hdfs/*",
        "$HADOOP_HDFS_HOME/share/hadoop/hdfs/lib/*",
        "$YARN_HOME/modules/*",
        "$YARN_HOME/lib/*"
      };

Not all deployment scenarii fit in this layout and therefore we need a standardized way to customize this class path.