Fix for doc formatting bug and small update to ISO build doc

Closes-Bug: #1260246

We need to remove nodejs and nodejs-legacy packages before installing nodejs
from the PPA repo in order to avoid conflicts with old system packages.
We also should not try to install "npm" package since it's a part of new nodejs

Change-Id: I5a5386317ba3ab8915bf0da089eab12e499fdb5a