Add attribute and configuration for show_image_direct_url

- Add default["openstack"]["image"]["api"]["show_image_direct_url"]
  which defaults to "False"
- Add option into glance-api.conf.erb
- Added test for show_image_direct_url setting
- Adjusted README to reference attribute

Change-Id: I83cd13db1d102118d696d371d94197044abf8e94
Closes-Bug: #1274960