nodepool: Allow one more node

Allow one more node so all jobs for pipeline check for openstack/cinder
can run in parallel.

Change-Id: Ic9e3e6608634d3b539cb02252ea98061e42d83f7
diff --git a/nodepool/nodepool.yaml b/nodepool/nodepool.yaml
index 3ebe01e..b0017c1 100644
--- a/nodepool/nodepool.yaml
+++ b/nodepool/nodepool.yaml
@@ -63,7 +63,7 @@
         username: ubuntu
     pools:
       - name: main
-        max-servers: 3
+        max-servers: 4
         auto-floating-ip: False
         networks:
           - lab-vlan400