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"