In k8s, use a secret instead of an env var for NEO4J_AUTH

[kdmccormick]

NEO4J_AUTH is passed into k8s-managed pods containers as an environment variable: https://github.com/openedx/tutor-contrib-coursegraph/blob/master/tutorcoursegraph/patches/k8s-deployments#L21-L27

This is a not a serious security issue, but it's not great either. Particularly, someone who has permissions to inspect a pod would be able to see the value of NEO4J_AUTH in plaintext.

Instead, the value should be passed into the container as a secret: https://kubernetes.io/docs/concepts/configuration/secret/

[kdmccormick]

@bmtcril , @feanil gave the plugin a look, and this is the one piece of feedback he had.

[bmtcril]

Nice, thanks for pointing that out!