Address code review comments:
1. Fix variable shadowing in collectEcVolumeServersByDc():
- Rename loop variable 'diskType' to 'diskTypeKey' and 'diskTypeStr'
to avoid shadowing the function parameter
2. Fix hardcoded HardDriveType in ecBalancer methods:
- balanceEcRack(): use ecb.diskType instead of types.HardDriveType
- collectVolumeIdToEcNodes(): use ecb.diskType
3. Add -diskType flag to ec.rebuild command:
- Add diskType field to ecRebuilder struct
- Pass diskType to collectEcNodes() and addEcVolumeShards()
4. Add -diskType flag to volumeServer.evacuate command:
- Add diskType field to commandVolumeServerEvacuate struct
- Pass diskType to collectEcVolumeServersByDc() and moveMountedShardToEcNode()