Fixed backups GET and DELETE to restrict access to the owner of backup

Fixed the db model find_by so that when a model instance is looked up and a
context is passed in, the find_by operation fails if the tenant_id in the
instance is not the same as the tenant_id in the context (except if the
tenant is an admin).

Fixed bug 1188822

Change-Id: I0374f999601fa3f2bc817901bbc428b8be219683
Merge "Fixed backups GET and DELETE to restrict access to the owner of backup"