IcingaConfig: Add v1-masterless config format

Do not render a passive checkable to the master zone then.

refs #12919

application/forms/SettingsForm.php

@@ -88,6 +88,7 @@ class SettingsForm extends QuickForm
                 array(
                     'v2' => $this->translate('Icinga v2.x'),
                     'v1' => $this->translate('Icinga v1.x'),
+                    'v1-masterless' => $this->translate('Icinga v1.x (no master)'),
                 )
             ),
             'description'  => $this->translate(

library/Director/IcingaConfig/IcingaConfig.php

@@ -90,7 +90,7 @@ class IcingaConfig
 
     public function isLegacy()
     {
-        return $this->configFormat === 'v1';
+        return strpos($this->configFormat, 'v1') === 0;
     }
 
     public function getObjectCount()

library/Director/Objects/IcingaObject.php

@@ -1482,6 +1482,7 @@ abstract class IcingaObject extends DbObject implements IcingaConfigRenderer
         if (
             $this->getResolvedProperty('zone_id')
             && array_key_exists('enable_active_checks', $this->defaultProperties)
+            && $config->getConfigFormat() !== 'v1-masterless'
         ) {
             $passive = clone($this);
             $passive->enable_active_checks = false;