loop over dnr nodes by topological sort

* Perform topological sort on graph nodes before looping over them to
mark do not run. This guarantees that parent nodes will be processed
before calling dependent child nodes. The complexity of the sorting is
N. The complexity of marking the the nodes is N*V

awx/main/scheduler/dag_workflow.py

@@ -1,6 +1,5 @@
 
 # Python
-import copy
 from awx.main.models import (
     WorkflowJobTemplateNode,
     WorkflowJobNode,
@@ -201,10 +200,9 @@ class WorkflowDAG(SimpleDAG):
 
     def mark_dnr_nodes(self):
         root_nodes = self.get_root_nodes()
-        nodes = copy.copy(root_nodes)
         nodes_marked_do_not_run = []
 
-        for index, node in enumerate(nodes):
+        for node in self.sort_nodes_topological():
             obj = node['node_object']
 
             if obj.do_not_run is False and not obj.job:
@@ -217,7 +215,4 @@ class WorkflowDAG(SimpleDAG):
                         obj.do_not_run = True
                         nodes_marked_do_not_run.append(node)
 
-            nodes.extend(self.get_dependencies(obj, 'success_nodes') +
-                         self.get_dependencies(obj, 'failure_nodes') +
-                         self.get_dependencies(obj, 'always_nodes'))
         return [n['node_object'] for n in nodes_marked_do_not_run]