HDFS federation : Fix TestBackupNode and TestRefreshNamenodes test failure.

1. TestBackupNode fails when check point signature does not match the image signature. BackupNode image is missing block pool ID and cluster ID.
2. Fix TestRefreshNamenodes. The change is to set the right configuration (DFS_FEDERATION_NAMESERVICES) in the test and also refresh the datanodes with the right configuration, so that the configuration has all the namenode specific parameters set. Regarding changes in MiniDFSCluster, when a namenode is added, configuration is updated to include new nameServiceId in the list of nameservices. The configuration is also updated with to add namenode configuration for the newly added nameServiceId. This configuration is used to refresh datanodes (so that it can pick up new nameServiceId and the corresponding namenode information).

Contributed by Suresh Srinivas.