Install Guide: Clarify package installation

Clarify the package installation so that it's clear that the
instructions should be complete and no further tweaking is needed.

Change-Id: I0e829e5f569a47e92f3e83c3f44d98eb99ccd0e9
backport: havana
Closes-Bug: #1245928
(cherry picked from commit b1b2116bcf6427b89c3c5d82b2228ed164addab3)