Run repository retention either after a successful backup or from a
dedicated systemd timer. Clean up script generation with templates and
expand molecule test coverage.

BREAKING CHANGE: Aggregate backup scripts are no longer managed, and
state=preset now requires at least one readable included directory.

Allow removing configuration for repositories.

Adds option to choose a non-default borg backup user on the server.

Add borg_ssh_key_per_repo option to generate unique SSH keypairs per
(server, repo) combination. When enabled, each repository gets its own
authorized_keys entry, enabling:

- Independent --append-only settings per repository
- Per-repo storage quotas

- Add test systems for client/server setup
- Add tests for systemd service and timer files
- Add tests for SSH connectivity
- Add tests for security configurations
- Add tests for multi-instance backup scenarios

Systemd gives us the ability to monitor backup job status using existing
monitoring solutions (node exporter) and allows us greater control over
the scheduling of the backup jobs.

This introduces a breaking change that requires users to manually remove
the old repositories from the clients and redeploying them with the
role. You will have to remove the Cron job that was created by the
Ansible script, everything else will be overwritten with a run from the
newer version.

- name: Remove backup cron jobs
  ansible.builtin.cron:
    name: BORG (Application level backups)
    state: absent
  become: true
- name: Remove env for backup cron job
  ansible.builtin.cron:
    name: BORG_PASSPHRASE
    env: true
    state: absent
  become: true

Performing manual migrations on the Borg server is not required.

We now additionally support multiple Borg repositories per client host
using the `borg_backup_argument` variable.

Signed-off-by: Colin Wilk <colin.wilk@tum.de>

Fix linting errors that came up with the new rules

Main changes:
* Enforce YAML document start headers
* Enforce spacing with commas, colons and hyphens
* Enforce indentation
* Enforce use of quotes only then required
* Enforce use of single quotes over double quotes
* Enforce use of true/false over yes/no and derivatives
* [..] Other minor rules that were already followed

Signed-off-by: Colin Wilk <colin.wilk@tum.de>

Signed-off-by: Colin Wilk <colin.wilk@tum.de>