Config overhaul: lowercase for `DatabaseDriver`

[josecelano]

Parent issue: #401

We are normalizing all enum variants to lowercase. DatabaseDriver is not normalized yet.

pub enum DatabaseDriver {
    /// The Sqlite3 database driver.
    Sqlite3,
    /// The `MySQL` database driver.
    MySQL,
}

[josecelano]

Hi @da2ce7 I think we should have two types for this concept. The enum was added to the primitives crate because it's also used in the configuration. However, I think the configuration and the database domain should be decoupled. I would add a new enum in the configuration crate and I would move this to the database mod.