Both Horizon and the cinder client documents the size paramater in
gigabyes(GBs) but the API docs(both v1 and v2) list the size units as
gibibytes(GiBs). The correct unit should be gibibytes(GiBs). This
should help in clearing up any confusion on what units are used in
Cinder.
Closes-bug: 1511167
Change-Id: I42e15c18d1f084d435a2be9b343fb2f6c3cf377f