Use Ubuntu minimal image as advanced guest image

Ubuntu minimal is smaller than regular cloud image.
It should works fine with less memory and boot faster than in
the gates.

Change-Id: Ie281698ecef05fd7ddf831eabd0e0121ac477709
diff --git a/zuul.d/base.yaml b/zuul.d/base.yaml
index 880e64f..12408ea 100644
--- a/zuul.d/base.yaml
+++ b/zuul.d/base.yaml
@@ -110,9 +110,10 @@
       devstack_localrc:
         PHYSICAL_NETWORK: default
         CIRROS_VERSION: 0.5.1
-        IMAGE_URLS: https://cloud-images.ubuntu.com/releases/bionic/release/ubuntu-18.04-server-cloudimg-amd64.img
-        ADVANCED_IMAGE_NAME: ubuntu-18.04-server-cloudimg-amd64
-        ADVANCED_INSTANCE_TYPE: ntp_image_384M
+        IMAGE_URLS: https://cloud-images.ubuntu.com/minimal/releases/focal/release/ubuntu-20.04-minimal-cloudimg-amd64.img
+        ADVANCED_IMAGE_NAME: ubuntu-20.04-minimal-cloudimg-amd64
+        ADVANCED_INSTANCE_TYPE: ntp_image_256M
         ADVANCED_INSTANCE_USER: ubuntu
+        CUSTOMIZE_IMAGE: true
         BUILD_TIMEOUT: 784
       tempest_concurrency: 3  # out of 4