Improve build group definitions

[sjpb]

This PR reorganises configuration of build VM inventory groups and improves image build documentation:

• 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 during a non-stackhpc extra build
   c. The sssd, tuned, squid and raid roles do not re-run unecessarily during a stackhpc extra build
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

[sjpb]

Image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/18194342712

[sjpb]

Image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/18194836628

[sjpb]

Image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/18196826773

[sjpb]

Inventory groups in extrabuild runs above:
RL8:

==> openstack.openhpc:     "group_names": [
==> openstack.openhpc:         "builder",
==> openstack.openhpc:         "cuda",
==> openstack.openhpc:         "dnf_repos",
==> openstack.openhpc:         "doca",
==> openstack.openhpc:         "slurm_recompile"
==> openstack.openhpc:     ]
==> openstack.openhpc: }

RL9:

==> openstack.openhpc:     "group_names": [
==> openstack.openhpc:         "builder",
==> openstack.openhpc:         "cuda",
==> openstack.openhpc:         "dnf_repos",
==> openstack.openhpc:         "doca",
==> openstack.openhpc:         "lustre",
==> openstack.openhpc:         "slurm_recompile"
==> openstack.openhpc:     ]

Looks OK

[bertiethorpe]

LGTM. This is a much clearer way of doing extra builds imo!