OpenStack-Tempest No nw_info cache associated with instance

When using Tempest to test an OpenStack-Cloud, it is easy to run into this error

tempest.lib.exceptions.BadRequest: Bad request
Details: {u'message': u'No nw_info cache associated with instance', u'code': 400}

In fact, networks are not configured proper. Tempest needs a full configured network, connected to the external network, to get some things working. This must be available to all tempest projects/tenante, hence it is enough to create it for each, but use the same name. I created a heat stack for this, and called this network needed-for-tempest. .. code:

heat_template_version: 2014-10-16

description: >
  Setting up a private Network with an snat-router

parameters:
  public_net_id:
    type: string
    description: ID of public network for which floating IP addresses will be allocated

resources:

  ### Creates a Neutron network and subnet
  network:
    type: OS::Neutron::Net
    properties:
      name: needed-for-tempest

  subnet:
    type: OS::Neutron::Subnet
    properties:
      name: needed-for-tempest1
      dns_nameservers:
        - 37.123.105.116
        - 37.123.105.117
      network_id: { get_resource: network }
      ip_version: 4
      gateway_ip: 10.0.0.1
      cidr: 10.0.0.0/24
      allocation_pools:
        - { start: 10.0.0.230, end: 10.0.0.240 }

  router:
    type: OS::Neutron::Router
    properties:
      external_gateway_info: { network: { get_param: public_net_id } }

  router_subnet_connect:
    type: OS::Neutron::RouterInterface
    depends_on: [ subnet, router ]
    properties: 
      router_id: { get_resource: router }
      subnet: { get_resource: subnet }

Of cource, tempest.conf has to be edited as well:

[network]
fixed_network_name = needed-for-tempest