Temporarily stop using citycloud-la1

At the request of the City Network Hosting operations team, we're
temporarily suspending use of the la1 region until they're able to
provision sufficient additional capacity there.

Change-Id: I17f8d4b74f095a22958287a52768d8e982b50120

nodepool/nodepool.yaml

@@ -963,7 +963,7 @@ providers:
     cloud: citycloud
     api-timeout: 60
     boot-timeout: 120
-    max-servers: 50
+    max-servers: 0
     rate: 0.25
     image-type: 'qcow2'
     template-hostname: '{image.name}-{timestamp}'