Install Guide: Add section about passwords

Add a section documenting all the passwords used in this guide.

Make passwords consistent

Change-Id: Ib9dc8bc53e1ef74c188f1362b29d7001955597ca
Closes-Bug: #1259684
backport: havana
(cherry picked from commit 81854a75c00546a05380803b3806ac408f43c802)
Merge "Install Guide: Add section about passwords" into stable/havana