fix(secret): improve secret default behavior

[RyanHolstien]

Improves the default security of a non-specified randomly generated key for clusters.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[Gerrit-K]

I just stumbled over this one and I'm curious: is this the cause fix for the following log in the fronend?

Your secret key is very short, and may be vulnerable to dictionary attacks.  Your application may not be secure.
The application secret should ideally be 32 bytes of completely random input, encoded in base64.
To set the application secret, please read http://playframework.com/documentation/latest/ApplicationSecret

[pedro93]

Hello Gerrit, the pr should help with the message yes