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/routes/test/mail-test.js

@@ -239,7 +239,7 @@ describe('Mail API', function () {
                 callback(null, dnsAnswerQueue[hostname][type]);
             };
 
-            dkimDomain = 'cloudron._domainkey.' + DOMAIN_0.domain;
+            dkimDomain = 'admincom-cloudron._domainkey.' + DOMAIN_0.domain;
             spfDomain = DOMAIN_0.domain;
             mxDomain = DOMAIN_0.domain;
             dmarcDomain = '_dmarc.' + DOMAIN_0.domain;