Avoid using hard coded datanode data dirs in unit tests

There are a few unit tests that rely on hard-coded MiniDFSCluster data dir names.

• TestDataNodeVolumeFailureToleration
• TestDataNodeVolumeFailureReporting
• TestDiskBalancerCommand
• TestBlockStatsMXBean
• TestDataNodeVolumeMetrics
• TestDFSAdmin
• TestDataNodeHotSwapVolumes
• TestDataNodeVolumeFailure

This ticket is opened to use

MiniDFSCluster#getInstanceStorageDir(0, 1);

instead of like below

new File(cluster.getDataDirectory(), "data1");