Improved installation guide diagrams

I improved the neutron initial networks diagram in the
installation guide.

Change-Id: I52593d97fbbfe1f8439cbf68a5fdc7f5d19364f8
Implements: blueprint installation-guide-improvements
backport: juno
(cherry picked from commit 21fd8de93cb3754375eb5f5bb657c4c6d605fdb9)
Merge "Improved installation guide diagrams" into stable/juno