fix(ingest): Fix Relay auth issues and add e2e event ingestion test (#578)

This is a long-needed test that tests the whole pipeline from Nginx, Relay, to Kafka, and Snuba. The final missing piece is testing the Symbolicator integration.

This PR is also a follow up to #576 as it didn't solve the Relay issues fully (the earlier fix was a coincidence or is not as reliable as it seemed).

Fixes #486 (finally?).