Can not update 'SubnetId' for instance

[huangtianhua]

1. create a stack with a AWS::EC2::Instance
**************************
heat_template_version: 2015-04-30
parameters:
  subnet:
    type: string
resources:
  cv:
    type: AWS::EC2::Instance
    properties:
      InstanceType: 1
      ImageId: 0baba961-8724-406e-8bc0-5d7f9b2da068
      SubnetId: {get_param: subnet}
*****************************
2. update the stack with a new subnet, the stack is in UPDATE_COMPLETE, but in fact, the network info has not changed after update
3. in handle_update() of AWS::EC2::Instance, if the 'NetworkInterfaces' property is not changed, won't to rebuild the network for instance.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/207334

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/207334
Committed: https://git.openstack.org/cgit/openstack/heat/commit/?id=79883338d0844818c3848ab466721ffb93daffd9
Submitter: Jenkins
Branch: master

commit 79883338d0844818c3848ab466721ffb93daffd9
Author: huangtianhua <email address hidden>
Date: Thu Jul 30 16:26:53 2015 +0800

    Make sure 'SubnetId' update works for instance

    'SubnetId' is updatable property, but now the
    update doesn't works if only update 'SubnetId'
    without 'NetworkInterfaces'.

    Change-Id: I86a9ce608ce13931a35d2776c6df0637bb022949
    Closes-Bug: #1479650