Restructured and updated Neutron initial networks section

As part of the installation guide improvement project, I performed
the following operations on the Neutron initial networks section:

1) Added new diagram to help visualize network components and their
   locations within the environment.
2) Added explanation for each procedure.
3) Added use of 'openrc' files.
4) Clarified phrasing and wording.

Change-Id: I69946a009807681115ebe23c011495ffb8b9467e
Partial-Bug: #1291071
Implements: blueprint networking-install-guide-improvements
Merge "Restructured and updated Neutron initial networks section"