Keyerror when displaying Instances & Volumes

This is most likely Essex specific because the corresponding code has changed in Folsom.

Steps to reproduce:

    Create the user USER and give it admin privileges
    Make the user a member of tenantA
    Create a volume in tenantB
    Associate the volume to an instance in tenantB
    Login to the dashboard as USER
    Go to the Volume & Instance menu entry
    See the following error message, where the key matches the UUID of the volume.

      KeyError at /nova/instances_and_volumes/
      u'f67c2eb9-9863-44c0-addf-1338233d7b4c'

The corresponding code in horizon/dashboards/nova/instances_and_volumes/views.py is

            volumes = api.volume_list(self.request)
            instances = SortedDict([(inst.id, inst) for inst in
                                    self._get_instances()])
            for volume in volumes:
                for att in volume.attachments:
                    att['instance'] = instances[att['server_id']]

And my first guess is that api.volume_list(self.request) returns all the volumes when the user has admin privileges, while self._get_instances() only returns the instances of the current tenant. When trying to get the instance to which each volume is attached, the lookup fails every time the volume is attached to a tenant that is not the current tenant.