Fix e2e tests

[markdboyd]

Changes proposed in this pull request:

Fixes #97
Closes https://github.com/cloud-gov/private/issues/920

• Reduce number of instances of proxy application to 1. With more than one instance of the proxy application in use, session state was not properly sychronized across the instances, so the tests would sometimes fail because depending on which instance was handling a proxied request, it might not recognize the authenticated session and the request would be rejected
• Update test proxy to run with 4 workers and use the eventlet worker type
• Update README notes on running tests
• Add success/failure notifications to pipeline
• Update pipeline to use hardened playwright-python image for e2e tests
• Add enforcement of black formatting
• Refactor seed script for e2e tests to better handle errors
• Update e2e tests to use assertions with auto-retry built-in
• Disable UI elements in OpenSearch Dashboards that only appear sometimes and cause e2e test flakiness
• Update OpenSearch and OpenSearch Dashboards to use 2.12.0

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

No known direct security impact of these changes, just improving the overall reliability and testability of the authentication proxy that will be used for the new customer logs system