SAMZA-2746: Allow customizable log4j configuration file (apache#1613)

* SAMZA-2746: Allow customizable log4j configuration file

Feature: Allow custome log4j configuration file to use in run-class.sh
Changes: Check existence of bash varibles and defaults to log4j.xml/log4j2.xml if not set
Tests: Existing tests pass
API changes: N/A
Upgrade Instructions: N/A
Usage Instructions: Set LOG4J_FILE_NAME and/or LOG4J2_FILE_NAME to desired log4j configuration file before invoking run-class.sh

* Simplify assignment

* Use double quotes and :- instead of :=

Author: kw2542

samza-shell/src/main/bash/run-class.sh

@@ -39,8 +39,8 @@ fi
 HADOOP_YARN_HOME="${HADOOP_YARN_HOME:-$HOME/.samza}"
 HADOOP_CONF_DIR="${HADOOP_CONF_DIR:-$HADOOP_YARN_HOME/conf}"
 GC_LOG_ROTATION_OPTS="-XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=10241024"
-LOG4J_FILE_NAME="log4j.xml"
-LOG4J2_FILE_NAME="log4j2.xml"
+LOG4J_FILE_NAME="${LOG4J_FILE_NAME:-log4j.xml}"
+LOG4J2_FILE_NAME="${LOG4J2_FILE_NAME:-log4j2.xml}"
 BASE_LIB_DIR="$base_dir/lib"
 DEFAULT_LOG4J_FILE=$BASE_LIB_DIR/$LOG4J_FILE_NAME
 DEFAULT_LOG4J2_FILE=$BASE_LIB_DIR/$LOG4J2_FILE_NAME