Make throttle optional

Tasks did not have the concept of throttling prior to ansible 2.9 so if
we want to allow this to be re-used with older versions then we need to
skip the throttling code if task doesn't contain the throttle attribute.
This will help if we want to backport this for upgrades.

Change-Id: I01f6e3a69dec25ec69782c087d6b81ce3f33e6c0

tripleo_ansible/ansible_plugins/strategy/tripleo_free.py

@@ -161,14 +161,17 @@ class StrategyModule(BASE.TripleoBase):
         task_vars = self._variable_manager.get_vars(**vars_params)
         templar = Templar(loader=self._loader, variables=task_vars)
 
-        try:
-            throttle = int(templar.template(task.throttle))
-        except Exception as e:
-            raise AnsibleError("Failed to throttle: {}".format(e),
-                               obj=task._df,
-                               orig_exc=e)
-        if self._check_throttle(throttle, task):
-            raise TripleoFreeBreak()
+        # if task has a throttle attribute, check throttle e.g. ansible > 2.9
+        throttle = getattr(task, 'throttle', None)
+        if throttle is not None:
+            try:
+                throttle = int(templar.template(throttle))
+            except Exception as e:
+                raise AnsibleError("Failed to throttle: {}".format(e),
+                                   obj=task._df,
+                                   orig_exc=e)
+            if self._check_throttle(throttle, task):
+                raise TripleoFreeBreak()
 
         # _blocked_hosts is used in the base strategy to keep track of hosts in
         # that have tasks in queue