Rename env var `TORRUST_TRACKER_DATABASE` to `TORRUST_TRACKER_DATABASE_DRIVER`

[josecelano]

Discussed in #415

^{Originally posted by josecelano September 11, 2023}
Hi @da2ce7, I think the name TORRUST_TRACKER_DATABASE can be confusing. I thought it was the database name. In the tracker configuration file (and in the code), we use the word driver to refer to the type of database system the tracker uses.

In fact, that could be the prefix (TORRUST_TRACKER_DATABASE_*) for all options related to the database.

db_driver = "Sqlite3"
db_path = "./storage/database/data.db"
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