borg-backup: use unique mark in .ssh/config

This writes out the ssh config so the backup process uses the right
key/user.  Since we have a transition period where we have bup and
borg backups we need to make the borg config have unique markers, or
the two fight over the configuration block.

Change-Id: I5455da3f2829e2aa8e0c531193adbbeff4b4776d

playbooks/roles/borg-backup/tasks/main.yaml

@@ -33,6 +33,7 @@
       IdentityFile /root/.ssh/id_borg_backup_ed25519
       User {{ borg_username }}
     mode: 0600
+    marker: '# {mark} ANSIBLE MANAGED BLOCK borg-backup'
   with_inventory_hostnames: borg-backup-server
 
 - name: Generate borg_user info tuple