Update Tracker and Index configuration

[josecelano]

The tracker and Index config files have changed.

Tracker:

• Config overhaul: make some fields mandatory torrust-tracker#992
• Config overhaul: use an explicit section for metadata torrust-tracker#991

Index:

• Config overhaul: make some fields mandatory torrust-index#700
• Rename config option secret_key to user_claim_token_pepper torrust-index#699
• Improve metadata section in configuration torrust-index#698