Limited authority viewer unable to see image details without location field

[Feilong Wang]

Actually, it's an improvement of bug https://bugs.launchpad.net/glance/+bug/1231255. Though we have fix 1231255, I think there is remaining concern like below:

1. OpenStack has CONF.show_image_direct_url or CONF.show_multiple_locations enabled because it needs it for some Glance-Nova setup.
2. OpenStack also has users with viewer only authority that are not to be allowed to see the location. Viewer has these policy grants: "get_images", "get_image" to access image details, but does not have "get_image_location" to protect the security sensitive details in the location field.

To allow for this, could the code be changed to put this except around the location access like this:
         try:
            if CONF.show_multiple_locations and image.locations:
                image_view['locations'] = list(image.locations)
            if CONF.show_image_direct_url and image.locations:
                image_view['direct_url'] = image.locations[0]['url']
        except exception.Forbidden:
            # log exception as warning

This would allow the 'viewer' to get list of images and image details but would not let them see the location that they are not allowed to view. TBH, I'm not really sure if it's a good idea to swallow the Forbidden exception. But obviously, it's breaking user's capability of image list.