[Proposal] Simplify the default configuration of DoctrineBundle

[javiereguiluz]

This is the default config provided by Symfony Flex:

###> doctrine/doctrine-bundle ###
DB_DRIVER=pdo_mysql
DB_HOST=127.0.0.1
DB_PORT=
DB_NAME=symfony
DB_USER=root
DB_PASSWORD=
###< doctrine/doctrine-bundle ###

# /etc/packages/doctrine.yaml
doctrine:
    dbal:
        driver: "%env(DB_DRIVER)%"
        host: "%env(DB_HOST)%"
        port: "%env(DB_PORT)%"
        dbname: "%env(DB_NAME)%"
        user: "%env(DB_USER)%"
        password: "%env(DB_PASSWORD)%"
    # ...

But maybe now it's the perfect time to switch to the simpler and more modern DSN-based alternative (like we do in the Symfony Demo app: https://github.com/symfony/symfony-demo/blob/master/app/config/parameters.yml.dist#L20). The perceived complexity is reduced significantly:

###> doctrine/doctrine-bundle ###
DB_URL='mysql://root@127.0.0.1:3306/symfony'
###< doctrine/doctrine-bundle ###

# /etc/packages/doctrine.yaml
doctrine:
    dbal:
        url: "%env(DB_URL)%"
    # ...

Another improvement of DSN-based config is that switching from a database to another is easier. For example: switching to SQLite is as easy as changing the value of DB_URL. Before you had to comment/uncomment config options in /etc/packages/doctrine.yml:

###> doctrine/doctrine-bundle ###
DB_URL='sqlite:///%kernel.root_dir%/../var/data/symfony.sqlite'
###< doctrine/doctrine-bundle ###

[fabpot]

I kept the "old" way because changing just a couple of parameters like the password or the db name seems easier with several vars. But we can definitely switch to a simple DSN (which should then be done for Swiftmailer as well for consistency).

[fabpot]

After thinking about this one, I'm sold. It's indeed much better. I've updated the recipes accordingly.