exim: fix a couple of config bugs

USERDB is actually being used even in routers, as a precondition to
route to the IMAP server.

helo_data for wiki_mail was completely broken; the right-hand side is
not a list, so exim just considered it malformed. Instead of replacing
it with wiki-mail-eqiad and hardcoding eqiad there, replace is with the
reverse DNS of the outgoing DNS (but provide a fallback), which should
make it future-proof.

Change-Id: I4c5df15c501727d18e25dee2e0a5b51b64293f5e