Handle negative values in total*Used for Cinder absolute limits

In some cases negative values can be returned for resources currently
in use. When this occurs, the project overview page fails with an
error 500 due to an OverflowError, "cannot convert float infinity to
integer". Handle this case more gracefully.

Change-Id: Id1ad16eafdde906dc74bfafc7f72ea8d3bdcdc8f
Closes-Bug: #1386687
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 "Handle negative values in total*Used for Cinder absolute limits"
Merge "Clarify meaning of "Disk GB Hours" column"