only set become defaults at last possible moment

tasks were overriding commandline with their defaults, not with the
explicit setting, removed the setting of defaults from task init and
pushed down to play context at last possible moment.
fixes #13362

lib/ansible/playbook/become.py

@@ -90,16 +90,18 @@ class Become:
 
             display.deprecated("Instead of su/su_user, use become/become_user and set become_method to 'su' (default is sudo)")
 
-        # if we are becoming someone else, but some fields are unset,
-        # make sure they're initialized to the default config values
-        if ds.get('become', False):
-            if ds.get('become_method', None) is None:
-                ds['become_method'] = C.DEFAULT_BECOME_METHOD
-            if ds.get('become_user', None) is None:
-                ds['become_user'] = C.DEFAULT_BECOME_USER
 
         return ds
 
+    def set_become_defaults(self, become, become_method, become_user):
+        ''' if we are becoming someone else, but some fields are unset,
+            make sure they're initialized to the default config values  '''
+        if become:
+            if become_method is None:
+                become_method = C.DEFAULT_BECOME_METHOD
+            if become_user is None:
+                become_user = C.DEFAULT_BECOME_USER
+
     def _get_attr_become(self):
         '''
         Override for the 'become' getattr fetcher, used from Base.

lib/ansible/playbook/play_context.py

@@ -392,6 +392,9 @@ class PlayContext(Base):
         if new_info.no_log is None:
             new_info.no_log = C.DEFAULT_NO_LOG
 
+        # set become defaults if not previouslly set
+        task.set_become_defaults(new_info.become, new_info.become_method, new_info.become_user)
+
         return new_info
 
     def make_become_cmd(self, cmd, executable=None):