Feature: Allow getting secret data from existing Kubernetes Secrets

[jessebot]

This allows users to use an existing Kubernetes Secret for both the admin user (currently not used in the StatefulSet), the API Key Origins, and API Key Secret. Here are the new values.yaml values:

adminUser:
  existingSecret: "" # use an existing secret for admin user
  # key in existing secret
  secretKeys:
    name: "name"
    auth: "auth"
    password: "password"

appSettings:    
  existingSecret: "" # use an existing secret for api key origin and secret
  # keys in existing secret
  secretKeys:
    apiKeyorigin: ""
    apiKeysecret: "secret"

This would help with keeping plain text passwords outside of the values.yaml and instead have them fetched from Kubernetes Secrets.