exim: remove default value of outbound_ips

We have a conditional on whether it's empty, so an empty value will do
the right thing. The default value didn't make sense anyway, as it
completely ignores IPv6.

Change-Id: I44009b3e098fd8d83381dc1c17b836a161fe7708
exim: swap the meaning of enable_external_mail

enable_external_mail was very strange: a value of "true" actually meant
"no external mail" and vice-versa; swap its meaning, its default values
and its two callers (otrs, rt) from true to false.

Change-Id: I628ce96f10b2645e2aa8cecf7cd743977b770143