Don't load non-referenced resources from DB

During a convergence check operation, we should never load resources from
the database. However, all resources will inevitably be created whenever we
do stack[some_resource]. Previously, resources not represented in the
cached data (i.e. the ones we were least interested in) would get loaded
from the database when this occurred.

This change ensures that only the resource being checked is loaded from the
database, as intended.

The asserts added in master to prevent regressions are *not* included in
the backport, since they add risk without benefit to a stable branch.

Change-Id: I9e568640d776748a1c9f2950f8bb0a8cea325996
Closes-Bug: #1656429
(cherry picked from commit 4507322675)

heat/engine/resource.py

@@ -259,11 +259,11 @@ class Resource(object):
         self.current_template_id = None
         self.root_stack_id = None
 
-        if not stack.has_cache_data(name):
+        if stack.cache_data is None:
             resource = stack.db_resource_get(name)
             if resource:
                 self._load_data(resource)
-        else:
+        elif stack.has_cache_data(name):
             self.action = stack.cache_data[name]['action']
             self.status = stack.cache_data[name]['status']
             self.id = stack.cache_data[name]['id']

heat/engine/stack.py

@@ -1194,6 +1194,7 @@ class Stack(collections.Mapping):
         self.t = template
         self.reset_dependencies()
         self._resources = None
+        self.cache_data = None
 
         if action is not self.CREATE:
             self.updated_time = oslo_timeutils.utcnow()