Improve build group definitions

[sjpb]

TODO: Make sure the docs are really clear so people don't start doing fatimage builds instead of extrabuilds by accident, maybe lead with extrabuilds
TODO: Validate on inventory_groups to ensure it never contains control/login/compute??

This PR reorganises configuration of build VM inventory groups:

• For extra builds, the builder groups are now entirely defined using the Packer inventory_groups variable, rather than also having builder as a child of groups in environments/{common,site}/inventory/groups.
• A group fatimage is defined, and added as a child of groups in the site groups file, to define the default functionality enabled for fat image builds.
• Group dependencies have been modified so that dnf_repos group is only added when required.

This fixes a number of issues:

1. It is clearer which features are active for an extra build, and how to change this.
2. Extra builds can run with minimal functionality. Specifically:
   a. Ark credentials are not always required (because dnf_repos role is no longer unconditionally enabled)
   b. The gateway role does not re-run unnecessarily.
3. The features a "fat image build" enables are actually defined. By default, a site-specific fat image will have the same features enabled as a StackHPC build, but this is configurable via the site groups file.

[sjpb]

Fat image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/17800575916

[sjpb]

Fat image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/17801882422

[sjpb]

Fat image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/17802381929

failed on

RL8:
==> openstack.openhpc: "No package ceph-common-17.2.7 available.
RL9:
==> openstack.openhpc: "No package ceph-common-18.2.4 available."

maybe repos are messed up?

[sjpb]

Image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/17824680461