vary dkim selector per mail domain

this is required for the case where the domain is added on multiple
cloudrons. initially, the plan was to just vary this as a derivation
of the dashboard domain. but this will break existing installation (wildcard
and manual domain setups cannot be re-programmed automatically).

src/test/ldap-test.js

@@ -100,7 +100,7 @@ function setup(done) {
         database._clear.bind(null),
         ldapServer.start.bind(null),
         domains.add.bind(null, DOMAIN_0.domain, DOMAIN_0, AUDIT_SOURCE),
-        maildb.add.bind(null, DOMAIN_0.domain),
+        maildb.add.bind(null, DOMAIN_0.domain, {}),
         function (callback) {
             users.createOwner(USER_0.username, USER_0.password, USER_0.email, USER_0.displayName, AUDIT_SOURCE, function (error, result) {
                 if (error) return callback(error);