Make use of the msmtp mta configurable

defaults/main.yml

@@ -35,3 +35,4 @@ borgbackup_keep_daily: 21
 borgbackup_keep_weekly: 8
 borgbackup_keep_monthly: 6
 borgbackup_keep_yearly: 0
+borgbackup_use_msmtp: true

meta/main.yml

 ---
 dependencies:
   - borgbackup
-  - msmtp_relay
+  - role: msmtp_relay
+    when: borgbackup_use_msmtp