Fix issue 1826: delete of missing managed objects should throw error in dsconfig

This change fixes a bug in the admin framework where an attempt to delete a non-existent managed object would not throw a ManagedObjectNotFoundException. In addition, a "-f" (or "--force") option has been added to the dsconfig delete-xxx sub-command to simulate the "-f" usage supported in the Unix "rm" command. If the "-f" option is specified, then no error will be returned to the client if the named managed object or its parents do not exist.