WIP: Add support for appliances_task_mode

[sjpb]

Adds a new variable appliances_task_mode which controls which taskfiles are run from roles. This takes options main and configure at present, with the latter usable running on a "fat" image and skipping most install tasks.

Earlier versions provided a ~25% speedup in site.yml, reducing it from 7m15s to 5m21s.

It currently affects:

• eessi
• os-manila-mount (could be neater with some role refactoring)
• opensearch
• filebeat
• slurm_exporter
• mysql
• openhpc
• grafana
• podman
• firewalld
• fail2ban
• cuda
• OOD desktop
• OOD jupyter
• freeipa client

Currently does not affect:

• nfs
• slurm_openstack_tools
• openondemand server
• prometheus
• nodeexporter

This is set to configure for the stackhpc CI environment. It could be used in caas to make platform creation faster.

Note currently an appliances_task_mode: install is NOT supported, and ansible/fatimage.yml determines what portions of roles are run.

[sjpb]

CI failure above was b/c grafana repos (which are still enabled, despite not doing an install) started failing GPG keys.