bastion: fix delivering non contributors emails locally

Should fix:
https://pagure.io/fedora-infrastructure/issue/12361

Basically postfix default is:
local_recipient_maps = proxy:unix:passwd.byname $alias_maps
so if the user is a local user or an alias, it's valid.
However, sssd and ipa show all users (even ones with no
access to that host). This means we were accepting and delivering
(locally) emails for anyuser@fedoraproject.org.

Setting this to just $alias_maps will just treat aliases as valid
and ignore all the local users. This should be fine as we use
aliases to send even to root or other system users.

Signed-off-by: Kevin Fenzi <kevin@scrye.com>

roles/base/files/postfix/main.cf/main.cf.gateway

@@ -783,3 +783,10 @@ sender_dependent_relayhost_maps = hash:/etc/postfix/bysender
 # http://www.postfix.org/BDAT_README.html
 # but does so in a way which is noisy with logs. Turn it off without logging.
 smtpd_discard_ehlo_keywords = chunking, silent-discard
+
+# We don't want to deliver to local users at all here
+# because sssd/ldap shows all existing users as users, but
+# we only want 'contributors' to have aliases.
+# Non contibutor's should have their mail rejected as not existing.
+# contributors we want to send to their aliases.
+local_recipient_maps = $alias_maps