User Story: As a storage admin using RHEL 7.5, I want to deploy RHCS 3.1 on these systems. Master Document: RHEL Installation Guide
Closing as these changes were published along with the Ceph 3.1 bits yesterday for GA: https://access.redhat.com/documentation/en-us/red_hat_ceph_storage/3/html-single/installation_guide_for_red_hat_enterprise_linux/#operating-system-requirements-for-red-hat-ceph-storage-install