Update override values for Keycloak testing steps

I've finished the testing steps for Keycloak (see #120 (closed)). Now I need to consolidate the various overrides I used to get Keycloak, Gitlab, Sonarqube, and Mattermost running and document that in DEVELOPMENT_MAINTENANCE.md.

This will include adding values to enable Elasticsearch in Mattermost, which was not working until recently.