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
5 files changed