Build Airflow Slack connections from path variables

[stacimc]

Current Situation

Currently, the Airflow connections for the Slack hook are configured through the Admin UI in production and by environment variable locally. The connection URI looks is formatted like: https://hooks.slack.com/services/foo/bar/baz. In order for the host to be parsed correctly, the string must be url encoded when set via an environment variable (ie, https://hooks.slack.com%2Fservices%2Ffoo%2Fbar%2Fbaz. This is somewhat tedious when switching between local testing set ups, so for convenience we modify the Slack environment variables in the entrypoint script to encode them properly (#3805).

Suggested Improvement

Modifying the environment variables in the entrypoint script is not ideal and has led to confusion in the past (#3805). We could instead use an Airflow variable to define the Slack webhook URL and then use it to create the Connection in the SlackMessage class.

Benefit

• avoids confusing modification of environment variables
• allows the webhook url to be obfuscated in the Airflow UI (as part of a Variable, rather than the host component of a Connection)