move network isolation advice to network concepts

Advice about isolation and segmentation of Private and Public networks is not
specific to Neutron. The relevant bits were integrated into the definitions of
these networks in the Logical Networks section. The item about co-locating
Admin and Private networks was poorly worded and is no longer true, so it's
removed altogether. Reference to OVS rephrased so as not to imply that there
are other topologies that are not OVS based and are less optimized.

Change-Id: I3782ee12db1e7583f729fe6e2828c2eebc378794
(cherry picked from commit 3b67063764b1188afc08f1c86cabe157f9796f5a)