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

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>