Fix pipeline slack calls

Since upgrading and tweaking our pipelines, we see the following in pipeline builds that try to interact with Slack:

The message to slack still works, and the build isnt marked as a failure, so this isn't high risk at the moment. But should be cleaned up. I think we aren't supposed to set a baseurl when using a botuser. Start by playing with the global slack configuration settings we're setting, I think that's where the problem might be.