Added ability to run HDFS service only with Hadoop 2

* Now Sahara provision YARN and MapReduce configs only when user
  selected appropriate processes in node groups

* Extracted getting hostnames of provisioned services to separate
  method in plugin utils

Change-Id: I2819c05201e6b35e4e5c8e810d243cb0b271073f
Closes-Bug: #1324383
Merge "Added ability to run HDFS service only with Hadoop 2"