SPARK-1167: Remove metrics-ganglia from default build due to LGPL issues...

This patch removes Ganglia integration from the default build. It
allows users willing to link against LGPL code to use Ganglia
by adding build flags or linking against a new Spark artifact called
spark-ganglia-lgpl.

This brings Spark in line with the Apache policy on LGPL code
enumerated here:

https://www.apache.org/legal/3party.html#options-optional

Author: Patrick Wendell <pwendell@gmail.com>

Closes #108 from pwendell/ganglia and squashes the following commits:

326712a [Patrick Wendell] Responding to review feedback
5f28ee4 [Patrick Wendell] SPARK-1167: Remove metrics-ganglia from default build due to LGPL issues.