[SPARK-7598] [DEPLOY] Add aliveWorkers metrics in Master

In Spark Standalone setup, when some workers are DEAD, they will stay in master worker list for a while.
master.workers metrics for master is only showing the total number of workers, we need to monitor how many real ALIVE workers are there to ensure the cluster is healthy.

Author: Rex Xiong <pengx@microsoft.com>

Closes #6117 from twilightgod/add-aliveWorker-metrics and squashes the following commits:

6be69a5 [Rex Xiong] Fix comment for aliveWorkers metrics
a882f39 [Rex Xiong] Fix style for aliveWorkers metrics
38ce955 [Rex Xiong] Add aliveWorkers metrics in Master