Default User Credentials

[Seokyun-Ha]

Is your feature request related to a problem? Please describe.
On the Dathaub Official documenet, it suggests the create default user credential by hand.
https://datahubproject.io/docs/authentication/changing-default-credentials/

However, this manual task can be ignored and default Datahub master account's id/pw is too trivial, so Anonymouses or Attackers can easily enter the datahub frontend page.

Describe the solution you'd like
Set default user credentials directly in helm value file, without creating secret manually.

Describe alternatives you've considered
The credential can be repalced by extraEnv and lifecycle.postStart. However, it is environment variable, so it can be revealed as plaintext, and revealed when helm diff. Also, this method only support single user id/pw replacing.

  extraEnvs:
    - name: DATAHUB_MASTER_PASSWORD
      value: newpassword
  lifecycle:
    # To add a new user to datahub in JAAS config without mounting the user.props file
    postStart:
      exec:
        command: ["/bin/sh", "-c", "echo datahub:$DATAHUB_MASTER_PASSWORD > /datahub-frontend/conf/user.props"]

Additional context
https://datahubproject.io/docs/authentication/changing-default-credentials/

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