Extends SWIFT loopback device size to 2G

The volume-upload-image test requires a cinder volume to be
uploaded in glance, see: https://review.openstack.org/#/c/33999/
Given that glance is configured to use swift as backend, we
extend the swift loopback device from 1G to be 2G (with thanks to
fungi and zhiyan who helped troubleshoot this.

Change-Id: I108b4ffeafcb1983745cbb20e88b85393c31464e
Merge "Extends SWIFT loopback device size to 2G"