See #256.
Most, but not all steps in the keycloak_realm tasks skip checking SSL certificates.

Since e.g. in roles/keycloak_realm/tasks/main.yml the first step Generate keycloak auth token has set
validate_certs: false, but the next step Determine if realm exists does not, I assume this was a mistake?

I briefly checked the other tasks in roles/keycloak_realm/tasks/ and added validate_certs: false where it was missing.
With those changes, the tasks now work for my setup with self-signed certificates.