Set explicitly the AZ nodepool to spawn instances in

Change-Id: I40213d1bad5205363326c8ece101922ab43b3191
diff --git a/nodepool/nodepool.yaml b/nodepool/nodepool.yaml
index 56ba3de..de91e5e 100644
--- a/nodepool/nodepool.yaml
+++ b/nodepool/nodepool.yaml
@@ -74,6 +74,8 @@
       - name: main
         max-servers: 6
         auto-floating-ip: False
+        availability-zones:
+          - lab
         networks:
           - lab-untrusted-access
         security-groups: