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.