[stable/drupal] Adds extra environment variables to the container

[scottrigby]

@prydonius Now that #1232 is merged, some images need user defined environment variables.

BTW, this feels like something that could be generally useful for all charts. Perhaps something we could add to other existing stable/charts, as well as a new PR for chartutil. Any thoughts about that?

[k8s-ci-robot]

Hi @scottrigby. Thanks for your PR.

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[scottrigby]

Version is bumped to the next MINOR, same as #1375.

[prydonius]

/ok-to-test

[viglesiasce]

Closing in favor of #1449