blueprint for 5.0.0 policy-handler new PDP API

blueprint for 5.0.0 policy-handler new PDP API

- in R4 Dublin the policy-engine introduced a totally new API
- policy-handler now has a startup option to either use
  = the new PDP API (default)
  = or the old PDP API that was created-updated before the end of 2018
- to use the old PDP API requires changing either the etc/config.json
  or setting up a non-empty env var PDP_API_VERSION on startup:
    export PDP_API_VERSION=2018
    docker run ... -e PDP_API_VERSION ...

Change-Id: I92941e908849baa94454ae6b094754965a37d470
Signed-off-by: Alex Shatov <alexs@att.com>
Issue-ID: DCAEGEN2-1128