Allow setup to be run again as long as not-activated

this is useful if the activation fails somewhere mid-way and we don't
need to re-setup the cloudron all over

src/mail.js

@@ -8,6 +8,7 @@ exports = module.exports = {
     getDomain: getDomain,
     addDomain: addDomain,
     removeDomain: removeDomain,
+    clearDomains: clearDomains,
 
     setDnsRecords: setDnsRecords,
 
@@ -807,6 +808,16 @@ function removeDomain(domain, callback) {
     });
 }
 
+function clearDomains(callback) {
+    assert.strictEqual(typeof callback, 'function');
+
+    maildb.clear(function (error) {
+        if (error) return callback(new MailError(MailError.INTERNAL_ERROR, error));
+
+        callback();
+    });
+}
+
 function setMailFromValidation(domain, enabled, callback) {
     assert.strictEqual(typeof domain, 'string');
     assert.strictEqual(typeof enabled, 'boolean');