Use default cirros image to reap the benefits of infra cache

This patch uses DevStack's default.
The default is now 0.4.0.
This avoids cache miss and need to download from flaky mirror.
See [1] for details.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-January/thread.html

Change-Id: Ic4598fa5234fb75da99706649db40c5ab925aa27
diff --git a/.zuul.yaml b/.zuul.yaml
index e544bdb..5616b7b 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -487,9 +487,7 @@
       tempest_test_regex: ^neutron_tempest_plugin\.scenario
       devstack_localrc:
         PHYSICAL_NETWORK: default
-        DOWNLOAD_DEFAULT_IMAGES: false
-        IMAGE_URLS: "http://download.cirros-cloud.net/0.3.4/cirros-0.3.4-i386-disk.img,https://cloud-images.ubuntu.com/releases/xenial/release/ubuntu-16.04-server-cloudimg-amd64-disk1.img"
-        DEFAULT_IMAGE_NAME: cirros-0.3.4-i386-disk
+        IMAGE_URLS: https://cloud-images.ubuntu.com/releases/xenial/release/ubuntu-16.04-server-cloudimg-amd64-disk1.img
         ADVANCED_IMAGE_NAME: ubuntu-16.04-server-cloudimg-amd64-disk1
         ADVANCED_INSTANCE_TYPE: ds512M
         ADVANCED_INSTANCE_USER: ubuntu
@@ -769,9 +767,7 @@
         USE_PYTHON3: true
         NETWORK_API_EXTENSIONS: "{{ (network_api_extensions_common + network_api_extensions_dvr) | join(',') }}"
         PHYSICAL_NETWORK: default
-        DOWNLOAD_DEFAULT_IMAGES: false
-        IMAGE_URLS: "http://download.cirros-cloud.net/0.3.4/cirros-0.3.4-i386-disk.img,https://cloud-images.ubuntu.com/releases/xenial/release/ubuntu-16.04-server-cloudimg-amd64-disk1.img"
-        DEFAULT_IMAGE_NAME: cirros-0.3.4-i386-disk
+        IMAGE_URLS: https://cloud-images.ubuntu.com/releases/xenial/release/ubuntu-16.04-server-cloudimg-amd64-disk1.img
         ADVANCED_IMAGE_NAME: ubuntu-16.04-server-cloudimg-amd64-disk1
         ADVANCED_INSTANCE_TYPE: ds512M
         ADVANCED_INSTANCE_USER: ubuntu
@@ -940,8 +936,6 @@
     vars:
       devstack_localrc:
         DESIGNATE_BACKEND_DRIVER: bind9
-        DOWNLOAD_DEFAULT_IMAGES: false
-        IMAGE_URLS: http://download.cirros-cloud.net/0.3.4/cirros-0.3.4-i386-disk.img,
         Q_AGENT: openvswitch
         # In this job advanced image is not needed, so it's name should be
         # empty