Add ability to turn off policy preload

Add ability to turn off policy preload

You can now set environment variable to override whether or not the
policies are pre-loaded by the engines during docker image startup.

The README.md is updated with instructions. Each docker-compose has
a different default set.

The .env file is set to load policies by default to true. You can override
by either changing this file or setting the environment:

export PRELOAD_POLICIES=false

Issue-ID: POLICY-304
Change-Id: Ia96788ac3b8d47814eea1046d96c4e3e4a0b9664
Signed-off-by: Pamela Dragosh <pdragosh@research.att.com>