tests failing with :`Only volume-backed servers are allowed for flavors with zero disk on stein`

  Traceback (most recent call last):
    File "tempest/api/compute/servers/test_create_server_multi_nic.py", line 107, in test_verify_duplicate_network_nics
      networks=networks, wait_until='ACTIVE')
    File "tempest/api/compute/base.py", line 246, in create_test_server
      **kwargs)
    File "tempest/common/compute.py", line 204, in create_test_server
      **kwargs)
    File "tempest/lib/services/compute/servers_client.py", line 101, in create_server
      resp, body = self.post('servers', post_body)
    File "tempest/lib/common/rest_client.py", line 283, in post
      return self.request('POST', url, extra_headers, headers, body, chunked)
    File "tempest/lib/services/compute/base_compute_client.py", line 48, in request
      method, url, extra_headers, headers, body, chunked)
    File "tempest/lib/common/rest_client.py", line 679, in request
      self._error_checker(resp, resp_body)
    File "tempest/lib/common/rest_client.py", line 780, in _error_checker
      raise exceptions.Forbidden(resp_body, resp=resp)
  tempest.lib.exceptions.Forbidden: Forbidden
  Details: {u'message': u'Only volume-backed servers are allowed for flavors with zero disk.', u'code': 403}

I believe the issue is that we are creating servers with a zero disk flavor:

https://github.com/openstack/rally-openstack/blob/25f17ce17f7e57060a94b5be1de4caf23632316f/rally_openstack/verification/tempest/context.py#L264

and the policy has changed in stein:

    The defalut value for policy rule
    ``os_compute_api:servers:create:zero_disk_flavor`` has changed from
    ``rule:admin_or_owner`` to ``rule:admin_api`` which means that by default,
    users without the admin role will not be allowed to create servers using
    a flavor with ``disk=0`` *unless* they are creating a volume-backed server.
    If you have these kinds of flavors, you may need to take action or
    temporarily override the policy rule. Refer to
    `bug 1739646 <https://launchpad.net/bugs/1739646>`_ for more details.

The question is what should we set the disk size to be for these auto generated flavors?