Add config for skipping authors and changelog.

Swift does not want to autogenerate these - so allow them to configure
them off.

Change-Id: I815fcbf9ec8d20f4a1c6cb4e2fe68a2ba8d1e305
Reviewed-on: https://review.openstack.org/27519
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins
Default to single-version-externally-managed

The other thing is always confusing and never what people want.

Change-Id: I915f561da2f74048c24dd324dbe71b5446d39197
Reviewed-on: https://review.openstack.org/28060
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins