Improve install guide security content

I improved security content in the installation guide as
follows:

1) Renamed basic environment 'passwords' section to 'security' to
   generalize topic.
2) Generalized existing content.
3) Added content about administrative privilege requirements
   including potential interference with deployment automation
   tools.

Recommend backporting to Icehouse.

Change-Id: Ide9785728c7b52ee1dc59a533b3486b99ee11139
Closes-Bug: #1311426
backport: icehouse
Merge "Improve install guide security content"