Make all_parents_must_converge settable when creating node

When targeting, ../workflow_job_templates/id#/workflow_nodes/ endpoint,
user could not set all_parents_must_converge to true.

awx issue ansible#7063

awx/api/serializers.py

@@ -3600,7 +3600,7 @@ def validate(self, attrs):
             ujt = self.instance.unified_job_template
         if ujt is None:
             ret = {}
-            for fd in ('workflow_job_template', 'identifier'):
+            for fd in ('workflow_job_template', 'identifier', 'all_parents_must_converge'):
                 if fd in attrs:
                     ret[fd] = attrs[fd]
             return ret