Fix EZP-22279: Integrate storage engine configuration with DoctrineBundle connections

**Work in progress**
https://jira.ez.no/browse/EZP-22279

This PR adds native support of DoctrineBundle, for DBAL configuration (ORM **not included**).

Added notion of *repository* in the configuration.
Repositories are declared globally and used/specified at the SiteAccess level (or group).

A repository consists of an *engine* (to specify which storage engine to use), and config options for that engine.
In the case of `legacy`, aka LegacyStorageEngine, a Doctrine *connection* is specified, which is configured in DoctrineBundle.

Example of semantic configuration:

```yaml
# Using Doctrine default connection
doctrine:
    dbal:
        driver:   pdo_mysql
        host:     localhost
        port:     3306
        dbname:   ezdemo
        user:     root
        password: root
        charset:  UTF8
 
ezpublish:
    repositories:
        # Takes all default values (type: legacy, connection: default)
        main: ~
 
    system:
        ezdemo_group:
            # Optional, if not provided, will take the first defined repository by default
            repository: main
 
#####
 
# Using a named Doctrine connection
doctrine:
    dbal:
        default_connection:       default
        connections:
            default:
                dbname:           Symfony2
                user:             root
                password:         null
                host:             localhost
            my_connection:
                dbname:           customer
                user:             root
                password:         null
                host:             localhost
 
ezpublish:
    repositories:
        main:
            # legacy => Legacy storage engine
            type: legacy
            connection: my_connection
```