Add config key to mount external volumes into north tool

This basically readds a feature to mount external folders into a north tools container. It was removed during the refactoring of the north config (see b0e28fa7).

Example config (under north):

north:
  external_mounts:
    - host_path: /mnt/pcshare
      bind: pcshare
      mode: ro
    - host_path: /scratch/localDataStore:
      bind: localDataStore
      mode: rw