Corrects the help message for trove_volume_support

Reasons:
- trove_volume_support has wrong help message in cfg.py code.

Changes:
- Corrected the help message.

Change-Id: I96ce1fe669d52d5c3fec81dd51b80f1fbe01ebfd
Closes-Bug: #1262398
Merge "Corrects the help message for trove_volume_support"