AMBARI-5352. PIG_OPTS should be appended instead of being over written (aonishuk)

ambari-server/src/main/resources/stacks/HDP/2.0.6/services/PIG/package/templates/pig-env.sh.j2

@@ -35,5 +35,5 @@ JAVA_HOME={{java64_home}}
 HADOOP_HOME=${HADOOP_HOME:-{{hadoop_home}}}
 
 if [ -d "/usr/lib/tez" ]; then
-  PIG_OPTS="-Dmapreduce.framework.name=yarn"
+  PIG_OPTS="$PIG_OPTS -Dmapreduce.framework.name=yarn"
 fi