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/rocky_jobs.yaml b/zuul.d/rocky_jobs.yaml
index 5cb6125..83b2f26 100644
--- a/zuul.d/rocky_jobs.yaml
+++ b/zuul.d/rocky_jobs.yaml
@@ -441,7 +441,7 @@
         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
+        ADVANCED_INSTANCE_TYPE: ds512M
         ADVANCED_INSTANCE_USER: ubuntu
         BUILD_TIMEOUT: 784
         TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin