Docs: Fix class-attribute use omission

It was an intended extension point for SubtestDoc
instances to alter their configuration representation
if needed.  However this hook was never tied in to
the actual usage by mistake.

Signed-off-by: Chris Evich <cevich@redhat.com>
Merge pull request #399 from cevich/fix_configdocclass

Docs: Fix class-attribute use omission