Merge and intersect lists without using sets.

Using sets for these operations is dangerous because sets cannot contain
certain object types (such as lists) and their iteration order is
undefined.

Fixes #7596

lib/ansible/utils/__init__.py

@@ -1005,21 +1005,19 @@ def is_list_of_strings(items):
             return False
     return True
 
-def _listify(a):
-    if not isinstance(a, (list, tuple)):
-        return [a,]
-    else:
-        return a
-
 def list_union(a, b):
-    set_a = set(_listify(a))
-    set_b = set(_listify(b))
-    return list(set_a.union(set_b))
+    result = list(a)
+    for i in b:
+        if i not in result:
+            result.append(i)
+    return result
 
 def list_intersection(a, b):
-    set_a = set(_listify(a))
-    set_b = set(_listify(b))
-    return list(set_a.intersection(set_b))
+    result = []
+    for i in a:
+        if i in b:
+            result.append(i)
+    return result
 
 def safe_eval(expr, locals={}, include_exceptions=False):
     '''