Group storage and vmail user options at mailserver.storage

Create a nicer option structure that deals with the mail storage and its
owner, uid, group and gid. Also includes the directory layout as a
property of how mails are stored..

tests/internal.nix

@@ -109,8 +109,11 @@ in
             "user2@example.com" = "user1@example.com";
           };
 
-          vmailGroupName = "vmail";
-          vmailUID = 5000;
+          storage = {
+            gid = 5000;
+            group = "vmail";
+          };
+
           indexDir = "/var/lib/dovecot/indices";
 
           enableImap = false;
@@ -218,7 +221,7 @@ in
 
       with subtest("Check dovecot maildir and index locations"):
           # If these paths change we need a migration
-          machine.succeed("doveadm user -f home user1@example.com | grep ${nodes.machine.mailserver.mailDirectory}/example.com/user1")
+          machine.succeed("doveadm user -f home user1@example.com | grep ${nodes.machine.mailserver.storage.path}/example.com/user1")
           machine.succeed("doveadm user -f mail user1@example.com | grep 'maildir:~/mail:INDEX=${nodes.machine.mailserver.indexDir}/example.com/user1'")
 
       with subtest("mail to send only accounts is rejected"):