Change Snowflake rendezvous to use Azure

Because of legacy/trac#25804 (moved), the App Engine–based rendezvous doesn't work anymore. In legacy/trac#22782 (moved), we set up CloudFront and Azure ones. However, because of recent messages from CloudFront, it's probably better to use Azure.

This rendezvous is also pointed at a different broker (the standalone broker from legacy/trac#22874 (moved)). But the change will not be noticeable to users. We actually meant to switch to the other broker earlier.