[convergence] stack update rollback with some wrong attributes

[huangtianhua]

In convergence:
1. create a stack with tags [t1, t2], timeout_mins=20
2. update the stack with tags [t3, t4], timeout_mins=30, and enable rollback
3. something wrong happened when stack update, so the stack enter rollback
4. the stack was rollback success,
5. but stack-show we found that the tags is [t3, t4], and the timeout_mins=30