Use standard assertVolumeStatusWait for volume detachment

Custom volume state detection method in test_compute_with_volumes
is unreasonable for volume detachment. Because in contrast to
volume attachment the target volume state ('available') is set when
all detachment processes have been finished.

Change-Id: I6d3d1186e73175fb25e2204088866cf53f61f4d5
Related-Bug: #1355285
Merge "Use standard assertVolumeStatusWait for volume detachment"