Fix expected response code of show_image_file

According to https://developer.openstack.org/api-ref/image/v2/#download-binary-image-data,
normal response codes of interface show_image_file should
be [200, 204, 206], this is to fix it.

Change-Id: I6eef7231742d873a0e727c300a9ae7f5958ac4f8
Closes-Bug: #1756264
2 files changed