Allow reading credentials from secrets

[artesby]

possible duplicates #128

If i understand correctly, currently:

credentials for external S3 can only be passed explicitly in values.yaml:

externalS3:
  accessKey: ...
  secretKey: ...

credentials for external kafka can only be passed explicitly in configmap:

extraConfigFiles:
  user.yaml: |+
    kafka:
      saslUsername: ...
      saslPassword: ...

Would be great if we could read these credentials from secrets.