heat clients : Make heat-boto rollback disabled by default

The boto library always sends the DisableRollback parameter,
defaulted to False, which is the opposite of the engine
default now, so we have to set DisableRollback to True in
the boto client wrapper

ref bug 1131303

Change-Id: I4acbba7b37f24cb1a874292ff2323ae24b630edf

heat/cfn_client/boto_client.py

@@ -39,10 +39,9 @@ def describe_stacks(self, **kwargs):
         return super(BotoClient, self).describe_stacks(stack_name)
 
     def create_stack(self, **kwargs):
-        disable_rollback = False
-        if 'DisableRollback' in kwargs:
-            if str(kwargs['DisableRollback']).lower() == 'true':
-                disable_rollback = True
+        disable_rollback = True
+        if str(kwargs.get('DisableRollback', '')).lower() == 'false':
+            disable_rollback = False
 
         if 'TemplateUrl' in kwargs:
             return super(BotoClient, self).create_stack(