redhat-ci.yml: rename "ephemeral" to "secondary-disk"
redhat-ci.yml: rename "ephemeral" to "secondary-disk"

This field was renamed in redhat-ci to "secondary-disk" to make it more
intuitive.

Closes: #190
Approved by: jlebon

thescriptbender/container-storage-setup,shishir-a412ed/docker-storage-setup