Spurious error that deletion of OpenStack VM failed

In many, but not all, runs of PerfKit Benchmarker on OpenStack, the following error is reported:
`2016-03-10 03:55:46,244 73012e99 Thread-25 speccpu2006(1/1) ERROR Got exception running _DeleteResource: Deletion of DebianBasedOpenStackVirtualMachine failed.`
I think this error is generally accompanied by a later message:
`2016-03-10 03:56:04,206 73012e99 Thread-25 speccpu2006(1/1) INFO Instance not found, may have been already deleted`

The code that spuriously emits this error is as follows. In perfkitbenchmarker/resource.py, `_DeleteResource` contains:

```
self._Delete()
try:
if self._Exists():
raise errors.Resource.RetryableDeletionError(
'Deletion of %s failed.' % type(self).__name__)
```

The problem is that once `self._Delete()` has initiated the deletion of the VM, `self._Exists()` checks on the status of the VM and, depending on timing, could get a result such as this:

```
{"server": {"status": "ACTIVE", "updated": "2016-03-04T10:28:06Z", "hostId": "5949369eedb068d54b06da3abd5b84388d5bd42fcc4606f712752fe5", "addresses": {}, "links": [{"href": "http://X.XX.XX.XXX:8774/v2/4f299816e33f49c7ae3819ee2d7d19d9/servers/a81f5829-af4e-40c7-89ce-fe035bb56c53", "rel": "self"}, {"href": "http://X.XX.XX.XXX:8774/4f299816e33f49c7ae3819ee2d7d19d9/servers/a81f5829-af4e-40c7-89ce-fe035bb56c53", "rel": "bookmark"}], "key_name": "perfkit_key_0_81d13aef", "image": {"id": "5b1d5b74-5f84-4b1d-b43b-d6ab54ba5eb7", "links": [{"href": "http://X.XX.XX.XXX:8774/4f299816e33f49c7ae3819ee2d7d19d9/images/5b1d5b74-5f84-4b1d-b43b-d6ab54ba5eb7", "rel": "bookmark"}]}, "OS-EXT-STS:task_state": "deleting", "OS-EXT-STS:vm_state": "active", "OS-SRV-USG:launched_at": "2016-03-03T23:39:19.000000", "flavor": {"id": "5ffb215e-ad47-40a4-b2ae-cafdc3d517a7", "links": [{"href": "http://X.XX.XX.XXX:8774/4f299816e33f49c7ae3819ee2d7d19d9/flavors/5ffb215e-ad47-40a4-b2ae-cafdc3d517a7", "rel": "bookmark"}]}, "id": "a81f5829-af4e-40c7-89ce-fe035bb56c53", "security_groups": [{"name": "perfkit_sc_group"}], "OS-SRV-USG:terminated_at": null, "OS-EXT-AZ:availability_zone": "nova", "user_id": "69718eb945aa431095556fb221612511", "name": "perfkit_vm_0_81d13aef", "created": "2016-03-03T23:38:59Z", "tenant_id": "4f299816e33f49c7ae3819ee2d7d19d9", "OS-DCF:diskConfig": "MANUAL", "os-extended-volumes:volumes_attached": [{"id": "b40f822a-cf89-40b4-8628-6e4426d2287f"}], "accessIPv4": "", "accessIPv6": "", "progress": 0, "OS-EXT-STS:power_state": 1, "config_drive": "True", "metadata": {}}}
```

(note the "task state" indicated as "deleting") or a result such as this:

```
{"server": {"status": "DELETED", "updated": "2016-03-10T08:55:34Z", "hostId": "c9a3adc9a453678d05fff5d06edc23314319a1c6ef1578c527e9ec76", "addresses": {}, "links": [{"href": "http://X.XX.XX.XXX:8774/v2/4f299816e33f49c7ae3819ee2d7d19d9/servers/3cfa3353-df42-4221-9fb7-01c32be7cd96", "rel": "self"}, {"href": "http://X.XX.XX.XXX:8774/4f299816e33f49c7ae3819ee2d7d19d9/servers/3cfa3353-df42-4221-9fb7-01c32be7cd96", "rel": "bookmark"}], "key_name": "perfkit_key_0_73012e99", "image": {"id": "5b1d5b74-5f84-4b1d-b43b-d6ab54ba5eb7", "links": [{"href": "http://X.XX.1XX.XXX:8774/4f299816e33f49c7ae3819ee2d7d19d9/images/5b1d5b74-5f84-4b1d-b43b-d6ab54ba5eb7", "rel": "bookmark"}]}, "OS-EXT-STS:task_state": null, "OS-EXT-STS:vm_state": "deleted", "OS-SRV-USG:launched_at": "2016-03-09T22:27:57.000000", "flavor": {"id": "5ffb215e-ad47-40a4-b2ae-cafdc3d517a7", "links": [{"href": "http://9.26.19.211:8774/4f299816e33f49c7ae3819ee2d7d19d9/flavors/5ffb215e-ad47-40a4-b2ae-cafdc3d517a7", "rel": "bookmark"}]}, "id": "3cfa3353-df42-4221-9fb7-01c32be7cd96", "security_groups": [{"name": "perfkit_sc_group"}], "OS-SRV-USG:terminated_at": "2016-03-10T08:55:46.000000", "OS-EXT-AZ:availability_zone": "nova", "user_id": "69718eb945aa431095556fb221612511", "name": "perfkit_vm_0_73012e99", "created": "2016-03-09T22:27:36Z", "tenant_id": "4f299816e33f49c7ae3819ee2d7d19d9", "OS-DCF:diskConfig": "MANUAL", "os-extended-volumes:volumes_attached": [{"id": "22cc9c2b-fb1e-4613-ac90-d9e30553ef7f"}], "accessIPv4": "", "accessIPv6": "", "OS-EXT-STS:power_state": 0, "config_drive": "True", "metadata": {}}}
```

(note the "status" indicating "DELETED"). In either case, it is clear that the VM is in the process of being deleted. In either case, the error message is spuriously generated.