Don't load new copies of current resources

The Stack._find_filtered_resources() method returns Resource objects for
all resources associated with the Stack, regardless of whether they are
current (present in the template; latest version in the case of
convergence). To do this, it previously created a new Resource object
for every resource found in the database.

However, for those resources which *are* current this is unnecessary. We
can access the Resource object simply through self.resources. It turns
out this is necessary for obtaining the required_by list for legacy
stacks, because only the Resources obtained from self.resources also
appear in the Dependencies graph obtained from self.dependencies. The
required_by list is read when listing or showing resources, which would
either return an empty list or fail for legacy stacks.

This patch also makes the Resource.required_by() method more robust in
its error handling.

Change-Id: Id438336e5c88dc7c2d168ba01ee703faa17e8b8e
Closes-Bug: #1703703
Related-Bug: #1523748

functional/test_resources_list.py

@@ -41,3 +41,10 @@ class ResourcesList(functional_base.FunctionalTestsBase):
                                              filters={'name': 'test2'})
 
         self.assertEqual('CREATE_COMPLETE', test2.resource_status)
+
+    def test_required_by(self):
+        stack_identifier = self.stack_create(template=test_template_depend)
+        [test1] = self.client.resources.list(stack_identifier,
+                                             filters={'name': 'test1'})
+
+        self.assertEqual(['test2'], test1.required_by)