Add clearer help kwarg for max_volumes_per_user

Add same wording as comment in trove.conf.sample.

Change-Id: I0ad8ca2dd2d6aaa4b7012c491c4a467b6004e4cb
Closes-Bug: #1274608
Merge "Add clearer help kwarg for max_volumes_per_user"
Add instance metadata functionality to trove