Workaround Cinder "wishlist" bug 1255593 for multi-backend volume_clear

Cinder currently only applies the volume_clear setting from the DEFAULT
section of cinder.conf if you're using a single backend.

The Cinder team has determined this is a 'wishlist' item to propagate
volume_clear to each backend, but it does impact usability and
performance.

To improve the performance of running Tempest with multi-backends in the
gate, workaround the bug in devstack.

Related-Bug: #1255593

Change-Id: Ia0ff5422f53eeda9a3ac4336eefec3b9bdea6da2
Merge "Workaround Cinder "wishlist" bug 1255593 for multi-backend volume_clear"