Clarified example architectures

I clarified the example architectures section to cover the two primary
architectures assumed throughout the installation guide. This patch
fixes the immediate issue of operators not understanding assumptions
made by the guide. A future patch for this bug will include improved
text and diagrams based on example architectures chosen for Icehouse.

Change-Id: I2529f7a4a66c0e7442d675f65909579aba3a0ad6
backport: havana
Partial-Bug: #1263719
(cherry picked from commit 420ed1d2198af31abe73d36cbb5a14ab72f6e3bb)