Fix the documentation to show how to set a custom message
Fix the documentation to show how to set a custom message

I had added a feature to get a custom message from hello-openshift by
using an environment variable.  But when I documented it, I used oc
set env (which worked for me since I was using a dc to test, but with
a bare pod, it can not work).  So, this changes the documentation to
show how to set an env in the pod definition, and how to make use of
it.

Fixes https://github.com/openshift/origin/issues/11093

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