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_storage_quota variable to limit repository storage on the
borg server via --storage-quota option in authorized_keys.

When not using borg_ssh_key_per_repo, all repos for a host must share
the same quota setting (similar to --append-only). Per-repo SSH keys
enable independent quotas per repository.

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

Useful for letting systemd services not get marked as failed state when
borg throws warnings. Keeping default to only accept exit code 0 as a
successful state.

Related: PR #2 https://github.com/kliwniloc/ansible-role-borgbackup/pull/2