Attempting to use 'externalDb' and the associated arguments described in the docs [1][2]
fails.

The installation fails with Keycloak, it's logs show it does not appear to recognize the given chePostgresHostName:
....Caused by: java.net.UnknownHostException: postgres.....

org_v1_che_cr.yaml (used in installation) was given a known good value from a Service:
chePostgresHostName: 'rickspostgresql.rickspostgres.svc.cluster.local'

[1] https://access.redhat.com/documentation/en-us/red_hat_codeready_workspaces/2.0/html-single/installation_guide/index#literal_database_literal_configuration_settings_related_to_the_database_used_by_codeready_workspaces
[2] https://che.legionx.com/docs/che-7/external-database-setup/