Changed assert statement of test_volume_found

Reason:test_volume_found is failing for datastore other than msql
       As volume used for cassandra and others is less than
       threshold defined in test

Change:Changed the threshold check for used volume

Change-Id: Idcf29e8dc924f192fc96682eff05878e274df7ab
Closes-Bug: #1301826
Merge "Changed assert statement of test_volume_found"