v-m/vmagent: Start in parallel

As with AlertManager, the point of having multiple replicas of `vmagent`
is so that one is always running, even if the other fails.  Thus, we
want to start the pods in parallel so that if the first one does not
come up, the second one at least has a chance.

victoria-metrics/vmagent.yaml

@@ -138,3 +138,4 @@ spec:
           name: vmagent
       - name: tmpdata
         emptyDir: {}
+  podManagementPolicy: Parallel