Clarify meaning of "Disk GB Hours" column

Admin > Overview Usage table has a column called "Disk GB Hours".
The meaning of this column is unclear.

Co-Authored-By: Thiago Paiva <thiagop@lsd.ufcg.edu.br>

Change-Id: I17e4a3ce60c66804e98a18c33261881ea10964b8
Closes-Bug: #1267362
Merge "Clarify meaning of "Disk GB Hours" column"