Install Guide: Mark content properly as Debian only

Some Debian specific content was shown on all systems, add proper os
tag.
Also, wrap mediaobjects in informalfigures.

Change-Id: I83df8d98e7843fd6ee8ce7f0d295473dd79fdf04
backport: havana
(cherry picked from commit 8affa7cfed134fc7438e4e3cf1df660e28d30f58)