Config overhaul: make secrets mandatory

[josecelano]

Parent issue: #938
Relates to:

• Config overhaul: make some fields mandatory #938
• Config overhaul: make some fields mandatory torrust-index#673 (comment)
• Config overhaul: make secrets mandatory torrust-index#710

Remove default value for http_api.access_tokens.admin.

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