resources on target host aren't released if resize fails

[Lars]

Description
===========
If we try to resize a host to a new flavor and resize fails due to missing ssh host-keys. Then the resources aren't released on the target host.

Steps to reproduce
==================
1. Check output from placement API

Output from placement API before resize command:

HOST A:
{
    "resource_provider_generation": 997,
    "usages": {
        "DISK_GB": 0,
        "MEMORY_MB": 495616,
        "VCPU": 84
    }
}

HOST B:
{
    "resource_provider_generation": 33,
    "usages": {
        "DISK_GB": 0,
        "MEMORY_MB": 221184,
        "VCPU": 40
    }
}

2. Try to resize host and check resources from placement API:

This is the output after resize (to flavor with 24GB and 12CPUs) failed:
HOST B:
{
    "resource_provider_generation": 33,
    "usages": {
        "DISK_GB": 0,
        "MEMORY_MB": 245760,
        "VCPU": 52
    }
}

3. Delete VM an check resources again
After deleting the VM the resources have been released (on source and target host).

Expected result
===============
If resizing fails resources must be released on target host.

Actual result
=============
Resources aren't released on target host.

Environment
===========
1. Openstack version:
nova-api 2:16.0.3-0ubuntu1~cloud0
nova-common 2:16.0.3-0ubuntu1~cloud0
nova-conductor 2:16.0.3-0ubuntu1~cloud0
nova-consoleauth 2:16.0.3-0ubuntu1~cloud0
nova-novncproxy 2:16.0.3-0ubuntu1~cloud0
nova-placement-api 2:16.0.3-0ubuntu1~cloud0
nova-scheduler 2:16.0.3-0ubuntu1~cloud0
python-nova 2:16.0.3-0ubuntu1~cloud0
python-novaclient 2:9.1.0-0ubuntu1~cloud0

2. Hypervisor:
Libvirt + KVM