Add requirement for APIImpact flag

Adds a requirement for an APIImpact flag in
the commit message for a proposed spec if it proposes
changes to the Cinder REST API. This will make it
much easier for people such as the API WG who want to
review API changes across OpenStack to find and
review proposed API changes.

Change-Id: I0e899e20d1ec35c1384385add6e9c676bd6ae1f9
Merge "Add requirement for APIImpact flag"