Update the following functions to accept/use diskType parameter:
- findEcVolumeShards()
- addEcVolumeShards()
- deleteEcVolumeShards()
- moveMountedShardToEcNode()
- countShardsByRack()
- pickNEcShardsToMoveFrom()
All ecBalancer methods now use ecb.diskType instead of hardcoded
types.HardDriveType. Non-ecBalancer callers (like volumeServer.evacuate
and ec.rebuild) use types.HardDriveType as the default.
Update all test files to pass diskType where needed.