Explain requirements for Public and Floating IPs

This was originally written up as https://review.openstack.org/#/c/87739/,

To accomodate this material, I bumped the Logical Networks section
down a level into a separate file and put that plus this material
into the network-concepts subfile, both under the "Networking Concepts"
top-level header.

I incorporated the comments that were given to the initial CM.

Closes-Bug: 1308161

Change-Id: Iefc2deff829b9823703f535d26b7bdcd857a5fbc