Code refactor according to comments

conf/metrics.properties.template

core/src/main/scala/spark/deploy/master/Master.scala

core/src/main/scala/spark/deploy/master/MasterSource.scala

core/src/main/scala/spark/deploy/worker/Worker.scala

core/src/main/scala/spark/deploy/worker/WorkerSource.scala

core/src/main/scala/spark/executor/ExecutorSource.scala

core/src/main/scala/spark/metrics/MetricsConfig.scala

core/src/main/scala/spark/metrics/MetricsSystem.scala

core/src/main/scala/spark/metrics/sink/ConsoleSink.scala

core/src/main/scala/spark/metrics/sink/CsvSink.scala

core/src/main/scala/spark/scheduler/DAGSchedulerSource.scala

core/src/main/scala/spark/storage/BlockManagerSource.scala

core/src/test/resources/test_metrics_config.properties

core/src/test/resources/test_metrics_system.properties

core/src/test/scala/spark/metrics/MetricsConfigSuite.scala

core/src/test/scala/spark/metrics/MetricsSystemSuite.scala