Merge branch '1.6.1-SNAPSHOT' of https://git-wip-us.apache.org/repos/asf/accumulo into 1.6.1-SNAPSHOT
ACCUMULO-2876 Use site config to configure default VolumeManagerImpl

This changes VolumeManagerImpl.get() to use the site configuration instead of the
ZooKeeper-based system configuration. This seems to align better with what is intended,
and it also eliminates the risk of an infinite loop, where the system configuration
requires an HdfsZooInstance instance ID, which requires a volume manager.
Merge branch '1.6.1-SNAPSHOT'