arch_backup major fixes

1. Don't automount disks (dconf changes) this breaks backups
2. Rewrite the entire backup script to handle mounting, decrypting,
   many use cases, etc
3. Update main.yaml to not create mount location for backup drive,
   script will handle it

ansible/arch_docker/tasks/main.yml

@@ -20,4 +20,3 @@
     daemon_reload: yes
     enabled: yes
   become: yes
-  tags: backup