rewrite description of logical networks

This list was replicated several places throughout the docs.
I merged in information from those other lists and replaced
them with a link to this list.

0500-nic-usage.rst was out of place at the end of this section
  so I pulled the Neutron information (that was under "Technical
  Considerations" later in this doc) up so it follows this material
  and included this file after that material.  Additional reorganization
  of the networking docs is deferred until a later release, but this
  puts one piece of that project in its proper location.

  The "Technical Considerations" intro was pointless and superfluous
  so I deleted it and restructured the files a bit, so the 6050-neutron*
  file begins with the content that was previously in the neutron-intro/
  0100-neutron-vs-nova file.  6050 now includes the intro content for
  this section, followed by the include lines that were previously in
  the 0060-* file.

Moved "Storage" above "Logical Networks".  This puts the OVS stuff right
  after Network Topologies.  Ultimately, the whole doc will flow with
  Big Picture, Controllers, Compute nodes, Storage, Networking.

Change-Id: I5d202f8c314018a4a14c8e01d56af4be835b4ce3