Relative paths need to be qualified

Due to the change in borg URL in the config
`borgbackup_repo_path` will by default be treated
as an absolute path. Prepend with `./` for
relative paths.

defaults/main.yml

@@ -32,7 +32,7 @@ borgbackup_exclude_dirs:
   - sh:/home/*/**/bower_components
 
 borgbackup_pool: "/mnt/borgpool"
-borgbackup_repo_path: "backup"
+borgbackup_repo_path: "./backup"
 borgbackup_keep_within: 7d
 borgbackup_keep_daily: 21
 borgbackup_keep_weekly: 8