openstack: Wait for volume deletion
openstack: Wait for volume deletion

Fix bug where volume deletion might have taken longer to delete and
shows ERROR on logs.

GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker