allow secretKey ref for external cassandra

[davidhiebert]

What was changed

Allow for the specification of a secretKey.

Why?

The bitnami Cassandra helm chart creates the password with the cassandra-passsword key. This allows the user to split Cassandra out of the Temporal helm release.

Checklist

1. Closes n/a

2. How was this tested: helm template .

3. Any docs updates needed? n/a

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