[hotfix] extract backup-related provision-steps to own playbook

context:

• in team-friendo setup, we want to provision prod to act as a backup source (ie: a server that generates backups and copies them to a backup destination)
• but this is (1) sort of detailed to explain to non-TF users who might want to setup their own instance and (2) assumes they want to do backups the way we do

changes:

• instead, let's just extract all the code that provisions prod into a backup source to provision_backup_src.yml and rename the playbook that provisions the backup destination to provision_backup_dst.ytl.
• while we're at it: remove stray instances of hard-coding aguestuser as the ansible_user in some of our playbooks! (whoops!)