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
Addresses: OSSN 0025 (https://review.openstack.org/#/c/117928/)