Allow docker auth for smoke tests

[nhsieh]

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WHAT is this change about?

This passes through environment variables to the smoke tests for dockerhub authentication. Related to https://github.com/cloudfoundry/cf-for-k8s/issues/580.

Does this PR introduce a change to config/values.yml?

No

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[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/175858781

The labels on this github issue will be updated when the story is started.