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
9cb715a5e7