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
Test swift multi-tenant store add context

Add a test to verify that the context is being propagated properly by
checking that the token is being passed to swift.

This checks the full image add (upload) path.

Change-Id: I5a9c6ce5f84639afac88186cc608d8dbb4d8ecb7
Test swift multi-tenant store get context

Add a test to verify that the context is being propagated properly by
checking that the token is being passed to swift.

This checks the full image get (download) path (at least as much as is
possible with a unit test).

Change-Id: I66346ca87aca592ffc060d90b7ec8afc95a80259
Merge "Test swift multi-tenant store add context"
Merge "Test swift multi-tenant store get context"
Merge "Fix public image ACL in multi-tenant Swift mode"