Resolve "external backup on borg"

Closes #40 (closed)

roles/ynh_backup/defaults/borg.yml

----
-# https://github.com/borgbase/ansible-role-borgbackup
-borgmatic_config_name: "borg_remote_repo"
-borg_encryption_passphrase: "{{ borg_remote_repo_encryption_passphrase }}"
-borg_repository: "{{ borg_remote_repository }}"
-borg_retention_policy:
-  keep_daily: "{{ borg_number_days_to_keep }}"
-  keep_weekly: "{{ borg_number_weeks_to_keep }}"
-  keep_monthly: "{{ borg_number_months_to_keep }}"
-borgmatic_cron_name: borgmatic_froggit_remote_backup
-borgmatic_cron_hour: "6"
-borgmatic_cron_minute: "0"