Add magnum section to tempest.conf template

The magnum section is used to declare configuration specific information
used by magnum-tempest-plugin

The keys used are:

- nic_id, to indicate the external network
- image_id, to indicate the fedora-coreos image to be used
- flavor_id, the flavor id to use when creating clusters
- dns_nameserver, the upstream dns server IP.
- network_driver, the network driver to test (flannel).
- labels, to pass a custom (local) image registry.
- insecure_registry, to mark the custom image registry as http (instead
  of https).

zaza/openstack/charm_tests/tempest/templates/tempest_v3.j2

@@ -122,5 +122,19 @@ test_server_path = {{ workspace_path }}/test_server.bin
 provider = amphora
 {% endif %}
 
+{%- if 'magnum' in enabled_services %}
+[magnum]
+nic_id = {{ ext_net }}
+image_id = {{ fedora_coreos_id }}
+master_flavor_id = {{ flavor_ref }}
+flavor_id = {{ flavor_ref }}
+dns_nameserver = {{ test_name_server }}
+network_driver = flannel
+{%- if test_registry_prefix %}
+labels = container_infra_prefix:{{ test_registry_prefix }}
+insecure_registry = {{ test_registry_prefix }}
+{%- endif %}
+{%- endif %}
+
 [dns]
 nameservers = {{ test_name_server }}