REGISTRAR: use VO's to/from mail addresses as backup for group notifications.

- When group attributes with to/from mail adresses are not set, then try to use VOs
  attributes instead. If that fails too, use backup address perun@cesnet.cz from config.
Merge pull request #473 from zlamalp/reg

REGISTRAR: use VO's to/from mail addresses as backup for group notifications.