Configuration overhaul: rename env vars

[josecelano]

Parent issue: #586

TORRUST_INDEX_CONFIG             -> TORRUST_INDEX_CONFIG_TOML
TORRUST_INDEX_PATH_CONFIG        -> TORRUST_INDEX_CONFIG_TOML_PATH
TORRUST_INDEX_TRACKER_API_TOKEN  -> TORRUST_INDEX_CONFIG_OVERRIDE_TRACKER__TOKEN
TORRUST_INDEX_AUTH_SECRET_KEY    -> TORRUST_INDEX_CONFIG_OVERRIDE_AUTH__SECRET_KEY

For E2E tests, we should use the same env var names. I will try to use the same env var names. I don't know why I used different names. If it's not possible, rename to:

TORRUST_INDEX_E2E_CONFIG          -> TORRUST_INDEX_E2E_CONFIG_TOML
TORRUST_INDEX_E2E_PATH_CONFIG     -> TORRUST_INDEX_E2E_CONFIG_TOML_PATH