Install Guide: Neutron Edits

Some edits, done as part of https://review.openstack.org/#/c/54960/2
(which was a backport)

Change-Id: Ib0ab68be53d5d6716c5d5de8323873105097f7a9
backport: havana
(cherry picked from commit 95e0a76977c3b8ae7349ba7596c870d93ed2b9d0)