Do not attempt to delete non-existing pvc

When a user is being deleted (e.g. by the culler), a PVC with name
"claim-username" is also attempted to be deleted by default as part
of the delete_forever function. This implies an extra call to the
API server to delete a PVC that we know does not exist. With the
KubeSpawner's delete_pvc flag set to false, we prevent that extra
call.

swan/values.yaml

@@ -178,6 +178,8 @@ jupyterhub:
         http_timeout: 45
         start_timeout: 60
         consecutive_failure_limit: 0
+      KubeSpawner:
+        delete_pvc: False
       SwanKubeSpawner:
         # set home directory to EOS
         local_home: False