Upgrade from debian stretch to buster

GCE has deprecated the stretch images and they are no longer
available for booting new nodes.

Change-Id: I4ca35ef961e2c546a4f256c9b4711a74c1736bda
diff --git a/nodepool/nodepool.yaml b/nodepool/nodepool.yaml
index 22f6a3b..beca1bc 100644
--- a/nodepool/nodepool.yaml
+++ b/nodepool/nodepool.yaml
@@ -14,9 +14,9 @@
     zone: us-central1-a
     boot-timeout: 120
     cloud-images:
-      - name: debian-stretch
+      - name: debian-buster
         image-project: debian-cloud
-        image-family: debian-9
+        image-family: debian-10
         username: zuul
         key: ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDaTT3+Y6dIh221ZsQ0ki6VwGb6lzd1/CvIS8RX6ZNmxJTXS7UKlvhAVTbGQPtmjdIQbwUyEqZEvIm/bUBr7dNbyeMLiRiRtoN+u1Quq8OAes2N7nUKnjLjGjVnM2mvYHGWHRDbpAQCtmXtbiQkHkfLLzewb+MqntQZ57xap31qriTkv4rdnlMN4kElILb5E/8WOAAJXYrt3b1TrTXwZUx7is1OTa38I9jJTpUMCn2otWGH9LlfeUhz/z8ZPB21wgUZN9CF5NwwBanqCNXy0jHiumOGx9e7Hc7apt641KiDuRt0aWyE6No/aMe6hVYnFR3NMG3rBtortXl+R5birK8l zuul
     pools:
@@ -36,59 +36,59 @@
         # long enough that it will have been updated by then).
         host-key-checking: False
         labels:
-          - name: debian-stretch-8G
+          - name: debian-buster-8G
             instance-type: n1-standard-2
-            cloud-image: debian-stretch
+            cloud-image: debian-buster
             volume-size: 40
-          - name: debian-stretch-30G
+          - name: debian-buster-30G
             instance-type: n1-standard-8
-            cloud-image: debian-stretch
+            cloud-image: debian-buster
             volume-size: 40
-          - name: debian-stretch-60G
+          - name: debian-buster-60G
             instance-type: n1-standard-16
-            cloud-image: debian-stretch
+            cloud-image: debian-buster
             volume-size: 40
-          - name: debian-stretch-n1-standard-2
+          - name: debian-buster-n1-standard-2
             instance-type: n1-standard-2
-            cloud-image: debian-stretch
+            cloud-image: debian-buster
             volume-size: 40
-          - name: debian-stretch-n1-standard-8
+          - name: debian-buster-n1-standard-8
             instance-type: n1-standard-8
-            cloud-image: debian-stretch
+            cloud-image: debian-buster
             volume-size: 40
-          - name: debian-stretch-n1-standard-16
+          - name: debian-buster-n1-standard-16
             instance-type: n1-standard-16
-            cloud-image: debian-stretch
+            cloud-image: debian-buster
             volume-size: 40
-          - name: debian-stretch-e2-standard-2
+          - name: debian-buster-e2-standard-2
             instance-type: e2-standard-2
-            cloud-image: debian-stretch
+            cloud-image: debian-buster
             volume-size: 40
-          - name: debian-stretch-e2-standard-8
+          - name: debian-buster-e2-standard-8
             instance-type: e2-standard-8
-            cloud-image: debian-stretch
+            cloud-image: debian-buster
             volume-size: 40
-          - name: debian-stretch-e2-standard-16
+          - name: debian-buster-e2-standard-16
             instance-type: e2-standard-16
-            cloud-image: debian-stretch
+            cloud-image: debian-buster
             volume-size: 40
 
 labels:
-  - name: debian-stretch-8G
+  - name: debian-buster-8G
     min-ready: 0
-  - name: debian-stretch-30G
+  - name: debian-buster-30G
     min-ready: 0
-  - name: debian-stretch-60G
+  - name: debian-buster-60G
     min-ready: 0
-  - name: debian-stretch-n1-standard-2
+  - name: debian-buster-n1-standard-2
     min-ready: 0
-  - name: debian-stretch-n1-standard-8
+  - name: debian-buster-n1-standard-8
     min-ready: 0
-  - name: debian-stretch-n1-standard-16
+  - name: debian-buster-n1-standard-16
     min-ready: 0
-  - name: debian-stretch-e2-standard-2
+  - name: debian-buster-e2-standard-2
     min-ready: 0
-  - name: debian-stretch-e2-standard-8
+  - name: debian-buster-e2-standard-8
     min-ready: 0
-  - name: debian-stretch-e2-standard-16
+  - name: debian-buster-e2-standard-16
     min-ready: 0