ref(k8s_util): change debugKey to DEIS_DEBUG

[bacongobbler]

Summary of Changes

This environment variable is being introduced into the actual application build. causing libraries like the ruby gem hiredis to install differently because we are affecting the build environment.

Issue(s) that this PR Closes

Please list the issue(s) that this PR closes, similar to the below:

closes deis/workflow#178

Testing Instructions

Please provide a detailed list for how to test the changes in this PR.

1. Create a Deis Cluster with this change as well as deis/slugbuilder#70
2. Deploy https://github.com/matthewrudy/ruby-env.git

The app should be deployed. Before, it should fail if using a version of hiredis earlier than https://github.com/redis/hiredis/pull/418, which is what matthewrudy/ruby-env demonstrates.

Pull Request Hygiene TODOs

Please make sure the below checklist is complete.

• [x] Your pull request is concise and to the point (make another PR for refactoring nearby code)
• [x] Your commits are squashed into logical units of work
• [x] Your commits follow the commit style guidelines

[jchauncey]

@bacongobbler need to rebase this