Fix public image ACL in multi-tenant Swift mode

Currently the ACL to public Swift images is '.r:*'. This means that
anonymous users who have not authenticated may be able to access the
data of a public Swift image when multi-tenant mode is enabled.

Change to use the cross-tenant '*:*' ACL which requires an authenticated
user for access.

Note: This does not address authenticated users being able to download
public image data directly from Swift (potentially bypassing Glance's
'download_image' policy).

Change-Id: I1fa3297908ca4be517419e9460f056a09aa98ef0
Addresses: OSSN 0025 (https://review.openstack.org/#/c/117928/)
Closes-bug: #1354512
Merge "Fix public image ACL in multi-tenant Swift mode"