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 HTTP 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: Iff76f9dad9658c8ab015e05d3b64f5fade0f7673
Merge "Add requirement for APIImpact flag"