Config overhaul: make secrets mandatory

[josecelano]

Parent issue: #673
Relates to:

• Config overhaul: make some fields mandatory #673 (comment)
• Config overhaul: make secrets mandatory torrust-tracker#1006

Remove default value for these options:

• tracker.token
• auth.user_claim_token_pepper
• mail.smpt.credentials.password

The option settings.database.connect_url might also contain secrets when the DB driver is MySQL. However, the default driver is SQLite.

[josecelano]

I'm going to keep the default values for mail.smpt.credentials because it's an empty string, and it's only needed if your SMTP server requires authentication. For example, using mailcatcher you don't need to provide credentials.