Fix formatting in release notes files

The change I04af514bbb237f405eb1d31ec10e3f019c85a43e breaks the rtd
verify job[1] because of the incorrect formating of the prelude section.

The prelude section can not contain a list as all prelude section are
concatenated together when the release notes are built. Merging the
preludes into their respective feature sections ensures the contex of
the comments are preserved as otherwise they will appear at the top of
the documentation apart from any other information.

Ref: https://docs.openstack.org/reno/latest/user/examples.html

[1] https://jenkins.opendaylight.org/releng/job/lf-infra-global-jjb-rtd-verify-any/475/

Change-Id: I3d23679eb4740b5fdc22fc566811d06ac6128820
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>