Upgrade from Debian stretch to buster

The stretch images are no longer available in GCE.

Change-Id: I9417ee45d2d43721052ad3e35d395bad7b53cb6a

zuul.d/jobs.yaml

diff --git a/zuul.d/jobs.yaml b/zuul.d/jobs.yaml
index b051667..d31c152 100644
--- a/zuul.d/jobs.yaml
+++ b/zuul.d/jobs.yaml
@@ -56,7 +56,7 @@
       - zuul: opendev.org/zuul/zuul-jobs
     timeout: 1800
     post-timeout: 1800
-    nodeset: debian-stretch
+    nodeset: debian-buster
 
 # See the procedure described above "base" before making changes to
 # this job.
@@ -75,7 +75,7 @@
       - zuul: opendev.org/zuul/zuul-jobs
     timeout: 1800
     post-timeout: 1800
-    nodeset: debian-stretch
+    nodeset: debian-buster
 
 # A mutex (default semaphore max is 1) so that only one job at a time
 # is updating the plugins.json file.

zuul.d/nodesets.yaml

diff --git a/zuul.d/nodesets.yaml b/zuul.d/nodesets.yaml
index 07d9368..7dae389 100644
--- a/zuul.d/nodesets.yaml
+++ b/zuul.d/nodesets.yaml
@@ -1,11 +1,11 @@
 - nodeset:
-    name: debian-stretch
+    name: debian-buster
     nodes:
       - name: testnode
-        label: debian-stretch-e2-standard-8
+        label: debian-buster-e2-standard-8
 
 - nodeset:
-    name: debian-stretch-large
+    name: debian-buster-large
     nodes:
       - name: testnode
-        label: debian-stretch-e2-standard-16
+        label: debian-buster-e2-standard-16