nodepool: Attempt to limit Cinder jobs to 1 at a time

Change-Id: Ifa149a6d0bcd681b3c10868a4411c1fdcc7bad5c
diff --git a/nodepool/nodepool.yaml b/nodepool/nodepool.yaml
index 921d567..d32df06 100644
--- a/nodepool/nodepool.yaml
+++ b/nodepool/nodepool.yaml
@@ -70,7 +70,7 @@
         username: ubuntu
     pools:
       - name: main
-        max-servers: 4
+        max-servers: 3
         auto-floating-ip: False
         networks:
           - lab-vlan400