Fix a race condition in GCE’s list_nodes() Closes #727 Invoking GCE’s `list_nodes()` while some VMs are being shutdown can result in the following exception to be raised out of `list_nodes()`:
Fix a race condition in GCE’s list_nodes()
Closes #727
Invoking GCE’s `list_nodes()` while some VMs are being shutdown can result
in the following exception to be raised out of `list_nodes()`:

```
  File "/usr/lib/python2.7/site-packages/libcloud/compute/drivers/gce.py", line 1411, in list_nodes
    v.get('instances', [])]
  File "/usr/lib/python2.7/site-packages/libcloud/compute/drivers/gce.py", line 5065, in _to_node
    extra['boot_disk'] = self.ex_get_volume(bd['name'], bd['zone'])
  File "/usr/lib/python2.7/site-packages/libcloud/compute/drivers/gce.py", line 3982, in ex_get_volume
    response = self.connection.request(request, method='GET').object
  File "/usr/lib/python2.7/site-packages/libcloud/common/google.py", line 684, in request
    *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/libcloud/common/base.py", line 736, in request
    response = responseCls(**kwargs)
  File "/usr/lib/python2.7/site-packages/libcloud/common/base.py", line 119, in __init__
    self.object = self.parse_body()
  File "/usr/lib/python2.7/site-packages/libcloud/common/google.py", line 259, in parse_body
    raise ResourceNotFoundError(message, self.status, code)
libcloud.common.google.ResourceNotFoundError: {'domain': 'global', 'message': "The resource 'projects/lenaic/zones/europe-west1-c/disks/devops-reg' was not found", 'reason': 'notFound'}
```

The above error occurred while the `devops-reg` machine was being deleted.

The issue occurs when the following events happen in that order:

* [`list_nodes()` sends a request to list all the instances.](https://github.com/apache/libcloud/blob/trunk/libcloud/compute/drivers/gce.py#L1622)
  At this point, the `devops-reg` was still existing.
* The `devops-reg` instance is deleted.
* `list_nodes()` calls `_to_node` which calls [`ex_get_volume` which attempts to retrieve the information of the volumes](https://github.com/apache/libcloud/blob/trunk/libcloud/compute/drivers/gce.py#L4235)
  But, as the instance was deleted since it was listed, `ex_get_volume` raises a `ResourceNotFoundError` exception.

When this happens, we should simply discard the node that was deleted during the execution of `list_nodes()` and return the information about the other nodes.

ZuluPro/libcloud,erjohnso/libcloud,ByteInternet/libcloud,SecurityCompass/libcloud,erjohnso/libcloud,supertom/libcloud,t-tran/libcloud,erjohnso/libcloud,mathspace/libcloud,andrewsomething/libcloud,curoverse/libcloud,supertom/libcloud,mistio/libcloud,Scalr/libcloud,wuyuewen/libcloud,mistio/libcloud,vongazman/libcloud,NexusIS/libcloud,niteoweb/libcloud,DimensionDataCBUSydney/libcloud,mathspace/libcloud,wrigri/libcloud,lochiiconnectivity/libcloud,t-tran/libcloud,mgogoulos/libcloud,mgogoulos/libcloud,StackPointCloud/libcloud,techhat/libcloud,DimensionDataCBUSydney/libcloud,iPlantCollaborativeOpenSource/libcloud,pquentin/libcloud,mathspace/libcloud,techhat/libcloud,vongazman/libcloud,apache/libcloud,wrigri/libcloud,curoverse/libcloud,watermelo/libcloud,techhat/libcloud,DimensionDataCBUSydney/libcloud,samuelchong/libcloud,illfelder/libcloud,Kami/libcloud,pquentin/libcloud,andrewsomething/libcloud,iPlantCollaborativeOpenSource/libcloud,pquentin/libcloud,mistio/libcloud,SecurityCompass/libcloud,niteoweb/libcloud,vongazman/libcloud,wrigri/libcloud,curoverse/libcloud,StackPointCloud/libcloud,wuyuewen/libcloud,t-tran/libcloud,Scalr/libcloud,watermelo/libcloud,andrewsomething/libcloud,ByteInternet/libcloud,illfelder/libcloud,samuelchong/libcloud,Kami/libcloud,wuyuewen/libcloud,iPlantCollaborativeOpenSource/libcloud,supertom/libcloud,ZuluPro/libcloud,apache/libcloud,lochiiconnectivity/libcloud,StackPointCloud/libcloud,Scalr/libcloud,samuelchong/libcloud,mgogoulos/libcloud,lochiiconnectivity/libcloud,illfelder/libcloud,Kami/libcloud,ZuluPro/libcloud,SecurityCompass/libcloud,niteoweb/libcloud,apache/libcloud,NexusIS/libcloud,watermelo/libcloud,NexusIS/libcloud,ByteInternet/libcloud