Add plan-environment.yaml

Add plan-environment.yaml

This file is needed for plan import and export features. We want to enable the
user to store the selection of environment options, so that it can be
re-imported, and it does not have to be perfmed manually multiple times.

The plan create workflow will look into the Swift
container for this file, and import its contents into the Mistral
environment. Conversely, plan export will create this file from the Mistral
environment contents, so that it can later be re-imported.

For more information, see the related blueprint, and the spec at
https://specs.openstack.org/openstack/tripleo-specs/specs/ocata/gui-plan-import-export.html

Partially implements: blueprint enhance-plan-creation-with-plan-environment

Change-Id: I95e3e3a25104623d6fcf38e99403cebbd591b92d