Fix check for missing subheadings
Fix check for missing subheadings

If a spec heading is missing entirely, and subheadings are expected,
the heading was being used in a key lookup causing an exception.

Change-Id: I44fb2b9c701cf359433211e876d2f6b3d1dbec5c

bcornec/ironic-specs,NaohiroTamura/ironic-specs,openstack/ironic-specs,naterh/ironic-specs