Correct the maintenance event types (#1830)

TERMINATE_ON_HOST_MAINTENANCE is the correct value that customers should expect. SHUTDOWN_ON_HOST_MAINTENANCE has never been a valid event type.

Author: dwightjl

compute/metadata/main.py

@@ -57,7 +57,7 @@ def wait_for_maintenance(callback):
         else:
             # Possible events:
             #   MIGRATE_ON_HOST_MAINTENANCE: instance will be migrated
-            #   SHUTDOWN_ON_HOST_MAINTENANCE: instance will be shut down
+            #   TERMINATE_ON_HOST_MAINTENANCE: instance will be shut down
             maintenance_event = r.text
 
         if maintenance_event != last_maintenance_event: