Limit main-zaza server instances to 1

We currently support one Zaza setup running at a time.

Change-Id: I1f7d9ec17aed7344004e5ad3bad685762b77dd9d
diff --git a/nodepool/nodepool.yaml b/nodepool/nodepool.yaml
index 9463b4b..e197857 100644
--- a/nodepool/nodepool.yaml
+++ b/nodepool/nodepool.yaml
@@ -69,6 +69,7 @@
             boot-from-volume: True
             volume-size: 80
       - name: zaza-main
+        max-servers: 1
         auto-floating-ip: False
         security-groups:
           - pp-net