Comment 3 for bug 1751923

This fix is in conflict with bugfix: 46922068ac167f492dd303efb359d0c649d69118.
We need to think twice how to fix it.

commit 46922068ac167f492dd303efb359d0c649d69118
Author: Aaron Rosen <email address hidden>
Date: Thu Dec 5 17:28:17 2013 -0800

    Make network_cache more robust with neutron

    Currently, nova treats neutron as the source of truth for which ports are
    attached to an instance which is a false assumption. Because of this
    if someone creates a port in neutron with a device_id that matches one
    of their existing instance_ids that port will eventually show up in
    nova list (through the periodic heal task).

    This problem usually manifests it's self when nova-compute
    calls to neutron to create a port and the request times out (though
    the port is actually created in neutron). When this occurs the instance
    can be rescheduled on another compute node which it will call out to
    neutron again to create a port. In this case two ports will show
    up in the network_cache table (since they have the same instance_id) though
    only one port is attached to the instance.

    This patch addresses this issue by only adding ports to network_cache
    if nova successfully allocated the port (or it was passed in). This
    way these ghost ports are avoided. A follow up patch will come later
    that garbage collects these ports.

    Closes-bug: #1258620
    Closes-bug: #1272195

    Change-Id: I961c224d95291727c8614174de07805a0d0a9e46