[SPARK-5078] Optionally read from SPARK_LOCAL_HOSTNAME

Current spark lets you set the ip address using SPARK_LOCAL_IP, but then this is given to akka after doing a reverse DNS lookup. This makes it difficult to run spark in Docker. You can already change the hostname that is used programmatically, but it would be nice to be able to do this with an environment variable as well.

Author: Michael Armbrust <michael@databricks.com>

Closes #3893 from marmbrus/localHostnameEnv and squashes the following commits:

85045b6 [Michael Armbrust] Optionally read from SPARK_LOCAL_HOSTNAME