Properly locate credentials in collection caches in mechglue
    
    Previously, we would just put the credentials in the default cache for
    a collection type, which lead to some mysterious failures.
    
    Signed-off-by: Robbie Harwood <rharwood@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>
    Merges: #221