This cartridge is documented in the Cartridge Guide.
rhc cartridge add -a myapp https://reflector-getupcloud.getup.io/reflect?github=dottobr83/openshift-cartridge-mariadb
This cartridge was extracted from the OpenShift Origin repository here: https://github.com/openshift/origin-server and can be installed as a downloadable cartridge. It exposes both OPENSHIFT_MARIADB and OPENSHIFT_MYSQL environment variables for compatibility.