Allows clusters to be deployed without outbound internet access. Key configuration changes:

• Some proxy variables are now defined for convenience in environments/common/inventory/group_vars/all/proxy.yml, making it easier to define a proxy using basic authentication.
• New variable proxy_remove allows removing proxy configuration at the end of Ansible runs. This defaults to false for clusters but true for build VMs. NB: This is a change in behaviour; before if the proxy group was enabled for the build then the proxy configuration would have been left in the image.
• New variable openondemand_install_apps_extra to simplify installing additional OpenOndemand Apps.

See docs/experimental/isolated-clusters.md for more information.

Design notes:

• All repos are now disabled at the end of both fatimage.yml site.yml playbooks via dnf_repos:disable_repos.yml. This means ansible.builtin.dnf tasks during site.yml do not trigger dnf cache updates and hence complete successfully (assuming relevant package(s) are actually installed).
• For supported features, installation tasks have been moved into fatimage.yml, with additional guards for appliances_mode added to avoid running them during site.yml.