the behaviours are different between orig and convg

[huangtianhua]

1. create a stack with template: http://paste.openstack.org/show/482482/
   heat stack-create -f /usr/hth/u_port.yaml C1 -P replacement_policy='REPLACE_ALWAYS'

2. update the stack with a new template: http://paste.openstack.org/show/482483/
   heat stack-update -f /usr/hth/pf.yaml -P replacement_policy='REPLACE_ALWAYS' c1 --rollback=true

3. the results of these steps: http://paste.openstack.org/show/482484/
we can find the port/subnet resources are keep the same after update, the port has been replaced by backup port resource when rollback.

4. set convergence_engine=true
5. delete the stack C1, then repeat the steps above, the results of these steps: http://paste.openstack.org/show/482485/
6. we can find the port has been replaced, not the backup port resource.

The behaviours are different in orig and convg.