Import the release-notes command from the release-tools repository and
integrate it with the list-changes command so that we see the release
notes for an upcoming release. This lets us review the release notes
output for warnings (like not having a documentation page linked) and
errors (such as reno failing to work).
Change-Id: I710606fe44601fe5414bfc82f25894319f16e558
Signed-off-by: Doug Hellmann <doug@doughellmann.com>