certificate scheme 2

On the fly create certificates via openssl (Maybe change this to
libressl in the future?). This is probably the best scheme to get
something that simply works. Self signed certificates only pose a
problem when connecting to retrieve the email via imap or pop3.

mail-server/postfix.nix

@@ -14,7 +14,7 @@
 #  You should have received a copy of the GNU General Public License
 #  along with this program. If not, see <http://www.gnu.org/licenses/>
 
-{ mail_dir, domain, valiases, certificate_scheme, cert_file, key_file }:
+{ mail_dir, domain, valiases, cert, key }:
 
 let
   # valiasToString :: { from = "..."; to = "..." } -> String
@@ -34,16 +34,6 @@ let
   # vhosts_file :: Path
   vhosts_file = builtins.toFile "vhosts" domain;
 
-  # cert :: PATH
-  cert = if certificate_scheme == 1
-         then cert_file
-         else "";
-
-  # key :: PATH
-  key = if certificate_scheme == 1
-        then key_file
-        else "";
-
 in
 {
   enable = true;