Create a 384MB flavor for advance image

In order to reduce the CI memory consuption, this patch creates a 384MB
flavor to replace the 512MB previously used for the advance image
(Ubuntu 18.04).

Flavor specs:
- 384MB RAM: aroung 100MB are used when the VM is started.
- 4GB disk: around 1,1GB are used when the VM is started.
- 1 VCPU.

Change-Id: I267a848d57183ddc39128f12d40b076360dfd3bc
Partial-Bug: #1940243
diff --git a/zuul.d/rocky_jobs.yaml b/zuul.d/rocky_jobs.yaml
index 11e4c9a..47f88f2 100644
--- a/zuul.d/rocky_jobs.yaml
+++ b/zuul.d/rocky_jobs.yaml
@@ -435,7 +435,7 @@
         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: ds512M
+        ADVANCED_INSTANCE_TYPE: ntp_image_384M
         ADVANCED_INSTANCE_USER: ubuntu
         BUILD_TIMEOUT: 784
         TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin