move from with_<lookup>: to loop:

- old functionality is still available direct lookup use, the following are equivalent

  with_nested: [[1,2,3], ['a','b','c']]

  loop: "{{lookup('nested', [1,2,3], ['a','b','c'])}}"

- avoid squashing with 'loop:'
- fixed test to use new intenal attributes
- removed most of 'lookup docs' as these now reside in the plugins

test/units/executor/test_task_executor.py

@@ -106,8 +106,8 @@ class TestTaskExecutor(unittest.TestCase):
         mock_host = MagicMock()
 
         mock_task = MagicMock()
-        mock_task.loop = 'items'
-        mock_task.loop_args = ['a', 'b', 'c']
+        mock_task.loop_with = 'items'
+        mock_task.loop = ['a', 'b', 'c']
 
         mock_play_context = MagicMock()