Increase the nodepool boot timeout

The default is 60 seconds, but we've seen GCE get into a cycle where
it takes longer; nodepool's quick retry in that case doesn't help.

Change-Id: I56f8c171f45bcf9950a7da6de63c4e5513dc5697
diff --git a/nodepool/nodepool.yaml b/nodepool/nodepool.yaml
index 6b2bc47..22f6a3b 100644
--- a/nodepool/nodepool.yaml
+++ b/nodepool/nodepool.yaml
@@ -12,6 +12,7 @@
     project: gerritcodereview-ci
     region: us-central1
     zone: us-central1-a
+    boot-timeout: 120
     cloud-images:
       - name: debian-stretch
         image-project: debian-cloud