data key should be stored as a secret

[dustinmm80]

The Conjur data key is currently set an an env var on the conjur pods. This is bad because the data key is shown in plaintext in k8s dashboard and API.

We should instead store this as a secret and reference it, like we do in the v5 oss helm chart.

https://github.com/cyberark/conjur-oss-helm-chart/blob/master/conjur-oss/templates/secrets.yaml#L2

This is a followup to PR #17.