Check status code when downloading empty file
Check status code when downloading empty file

According to
https://developer.openstack.org/api-ref/image/v2/#download-binary-image-data
The status code of Glance API is 204 when downloading empty file, so
this patch adds the corresponding check on a test.

This patch comes from I6eef7231742d873a0e727c300a9ae7f5958ac4f8

Change-Id: Ia45b584e3783c6ab6d7c94a817da9b5058f1c351
Related-Bug: #1756264

cisco-openstack/tempest,openstack/tempest,openstack/tempest,masayukig/tempest,masayukig/tempest,cisco-openstack/tempest