server networks update incorrect if enable observe_on_update

[huangtianhua]

observe_on_update = true

1. create a stack with nova server:
**************************
th_server:
    type: OS::Nova::Server
    properties:
      networks:
        - network: {get_param: net1}
***************************
2. update the stack with the same template, the server networks info changed, because we get the live networks data first and update the old_nets, then to match the old_nets and newnets, the result is they are not same.