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 get context"