Add missing mocked calls to the Volumes tests

There is a quota check for the Create Volume button to see if it
should be enabled. These new calls to tenant_quota_usages were missed
when the volume snapshots tests moved to the main volumes
directory/panel.

Change-Id: If09afd6d69a9a9b14dfb6d802be92b7e24e63c60
Closes-Bug: #1284664