Deprecate putting periodic tasks on a driver object

This approach will no longer make sense when the driver composition is in effect.
Anyway usually a better place to put a task is an interface.

Change-Id: I7096e428ce9774d89ac624c2d38bb23984a4b842
Related-Bug: #1524745

doc/source/dev/architecture.rst

@@ -76,8 +76,8 @@ Driver-Specific Periodic Tasks
 ------------------------------
 
 Drivers may run their own periodic tasks, i.e. actions run repeatedly after
-a certain amount of time. Such task is created by decorating a method on the
-driver itself or on any interface with periodic_ decorator, e.g.
+a certain amount of time. Such task is created by decorating a method on
+an interface with periodic_ decorator, e.g.
 
 ::
 
@@ -88,18 +88,15 @@ driver itself or on any interface with periodic_ decorator, e.g.
         def task(self, manager, context):
             pass  # do something
 
-    class FakeDriver(base.BaseDriver):
-        def __init__(self):
-            self.power = FakePower()
-
-        @periodics.periodic(spacing=42)
-        def task2(self, manager, context):
-            pass  # do something
-
 
 Here the ``spacing`` argument is a period in seconds for a given periodic task.
 For example 'spacing=5' means every 5 seconds.
 
+.. note::
+    As of the Newton release, it's possible to bind periodic tasks to a driver
+    object instead of an interface. This is deprecated and support for it will
+    be removed in the Ocata release.
+
 
 Message Routing
 ===============

ironic/conductor/base_manager.py

@@ -110,6 +110,8 @@ class BaseConductorManager(object):
         periodic_task_classes = set()
         self._collect_periodic_tasks(self, (admin_context,))
         for driver_obj in drivers.values():
+            # TODO(dtantsur): collecting tasks from driver objects is
+            # deprecated and should be removed in Ocata.
             self._collect_periodic_tasks(driver_obj, (self, admin_context))
             for iface_name in driver_obj.all_interfaces:
                 iface = getattr(driver_obj, iface_name, None)

releasenotes/notes/periodic-tasks-drivers-ae9cddab88b546c6.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - Putting periodic tasks on a driver object (rather than interface) is
+    deprecated. Driver developers should move periodic tasks from driver
+    objects to interface objects.