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 Trove 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: Ic3a6c3ef65fc0ccbdffe24903faf7c7a99d49d88
Merge "Add requirement for APIImpact flag"