Update README on how to get public cloud image names for VNF packages

  -h, --help                  Show this message and exit.

```

## Using images in public clouds

When a NF package wants to use an image from the public cloud, we need to specify the specific image in that public cloud. The name will be different for each public cloud. That's why we need to specify, for each VIM type, a different image. Find below an excerpt from `hackfest_basic_vnf`:

```yaml

  sw-image-desc:

  - id: ubuntu20.04

    image: ubuntu20.04

    name: ubuntu20.04

  - id: ubuntu20.04-aws

    name: ubuntu20.04-aws

    image: ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-20231025

    vim-type: aws

  - id: ubuntu20.04-azure

    name: ubuntu20.04-azure

    image: Canonical:0001-com-ubuntu-server-focal:20_04-lts:latest

    vim-type: azure

  - id: ubuntu20.04-gcp

    name: ubuntu20.04-gcp

    image: ubuntu-os-cloud:image-family:ubuntu-2004-lts

    vim-type: gcp

```

In order to know what are the name of the images on each public cloud, you could check the links below:

- Ubuntu images in Azure: <https://canonical-azure.readthedocs-hosted.com/en/latest/azure-how-to/instances/find-ubuntu-images/>

- Ubuntu images in GCP: <https://canonical-gcp.readthedocs-hosted.com/en/latest/google-how-to/gce/find-ubuntu-images/>

- Ubuntu images in AWS: <https://aws.amazon.com/marketplace/pp/prodview-tuap4sdmqpo52>

  - From AWS Console > EC2 > Images > AMI > Search

    - Public images

    - Filter: "Platform=Ubuntu", "Virtualization=hvm", "Architecture=x86_64", "AMI name=ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server"