MetricsServlet code refactor according to comments

conf/metrics.properties.template

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

...c/main/scala/spark/deploy/master/ui/ApplicationPage.scala

core/src/main/scala/spark/deploy/master/ui/IndexPage.scala

core/src/main/scala/spark/deploy/master/ui/MasterWebUI.scala

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

core/src/main/scala/spark/deploy/worker/ui/WorkerWebUI.scala

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

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

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

core/src/main/scala/spark/ui/SparkUI.scala

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