mistral-executor include host /etc/environment

The undercloud host file /etc/environment will be documented as where
proxy environment variables will be set when the undercloud is behind
a proxy server.

These variables also need to be set inside the mistral_executor
container for the pre-deployment image prepare to access external
registries. This change will make that possible without any further
changes required from the deployer.

Change-Id: Ie8924833a2410eea096fda955c8c6df9b1f0e207
Partial-Bug: #1818590

deployment/mistral/mistral-executor-container-puppet.yaml

@@ -200,6 +200,7 @@ outputs:
                         tripleo-admin: {get_param: TripleoAdminUser}
             environment:
               - KOLLA_CONFIG_STRATEGY=COPY_ALWAYS
+            env_file: /etc/environment
       host_prep_tasks:
         - import_role:
             name: tripleo-create-admin