Mention optional networks

I updated the text to mention optional network interfaces
and networks present in the improved diagrams. I also
mentioned the progression toward use of automation tools
for production environments.

Change-Id: I4546662b9d9602343f69b1f7d1a6f2092b0e403e
Implements: blueprint installation-guide-improvements
backport: juno
(cherry picked from commit 5dd620d59a93764cfc9d37a27c4a86f5186cd689)