Fixes LP #992640 - Volumes sometimes not cleaned
Fixes LP #992640 - Volumes sometimes not cleaned

* Adds some error handling to the volume list test case
  that properly handles the (fairly common) case for when
  Nova's volume group backing file size is too small to create
  the needed 3 1G volumes for the test

Change-Id: Ice367b73303d74c529351d7bd4d9a4e9b1348e07

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