Fix for HDFS on Swarm.

[timothymschier]

No description provided.

[timothymschier]

Was getting the following error on hdfs-name:
WARN blockmanagement.DatanodeManager: Unresolved datanode registration: hostname cannot be resolved (ip=10.42.20.13, hostname=10.42.20.13)

and the following error on hdfs-data:
ERROR datanode.DataNode: Initialization failed for Block pool BP-592837392-10.42.106.189-1480927625303 (Datanode Uuid null) service to hdfs-name/10.42.106.189:8020 Datanode denied communication with namenode because hostname cannot be resolved (ip=10.42.20.13, hostname=10.42.20.13): DatanodeRegistration(0.0.0.0:50010, datanodeUuid=0f7487b4-190f-4cab-bd3a-362065222af4, infoPort=50075, infoSecurePort=0, ipcPort=50020, storageInfo=lv=-56;cid=CID-b892f836-83ce-4c6c-9b97-991d1c7e1c80;nsid=589517046;c=0)

This change fixes both.