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.