def teardown(exceptions=[])
      stop

      (vm_groups.keys.to_set - exceptions.to_set).each do |group_name|
        deprovision_group(group_name) # clean this after everything finishes
      end
    end
def teardown(exceptions=[])
      stop

      (vm_groups.keys.to_set - exceptions.to_set).each do |group_name|
        deprovision_group(group_name) # clean this after everything finishes
      end
    end
Instruct all provisioners except ones in the exception list to tear down.
 Calls #stop as its first action.

 This is always done serially. For sanity.