scappy: light refactoring

* Add read_hosts helper for reading dsh host groups into a list.
* Add fix for find-nearest-rsync from Iafe5a1e27.
* Don't pass os.environ to subprocess.check_call; it does that by default when
  env is unspecified.

Change-Id: I9be6822a56381ff3b429025f87695926402923eb