v-m/alertmanager: Bring up replicas in parallel

If something prevents the first AlertManager instance from starting, we
don't want to wait forever for it before starting the second.  That
pretty much defeats the purpose of having two instances.  Fortunately,
we can configure Kubernetes to bring up both instances simultaneously by
setting the pod management policyo to `Parallel`.

victoria-metrics/alertmanager.yaml

@@ -70,6 +70,7 @@ spec:
       - name: config
         configMap:
           name: alertmanager
+  podManagementPolicy: Parallel
   volumeClaimTemplates:
   - apiVersion: v1
     kind: PersistentVolumeClaim