Improved installation guide diagrams

I improved the example architecture diagrams in the
installation guide. Future patches will address other
diagrams using a similar theme. This patch also includes
some content changes to agree with the new diagrams.

Change-Id: I5ba3d035805200716c1cebc1a0fedb0b959e6966
Implements: blueprint installation-guide-improvements
backport: juno
Merge "Improved installation guide diagrams"