[SPARK-21798]: No config to replace deprecated SPARK_CLASSPATH config for launching daemons like History Server

Adding new env variable SPARK_DAEMON_CLASSPATH to set classpath for launching daemons. Tested and verified for History Server and Standalone Mode.

Author: pgandhi

launcher/src/main/java/org/apache/spark/launcher/AbstractCommandBuilder.java

@@ -136,7 +136,8 @@ List<String> buildClassPath(String appClassPath) throws IOException {
 
     Set<String> cp = new LinkedHashSet<>();
     addToClassPath(cp, appClassPath);
-
+    addToClassPath(cp, getenv("SPARK_DAEMON_CLASSPATH"));
+    
     addToClassPath(cp, getConfDir());
 
     boolean prependClasses = !isEmpty(getenv("SPARK_PREPEND_CLASSES"));