Implemented EZP-20821: Make it possible to configure the front controller without editing it

Example of Apache configuration in the VirtualHost:

```
    # Environment.
    # Possible values: "prod", "dev", "staging",...
    # Defaults to "prod" if omitted
    SetEnv ENVIRONMENT "prod"

    # Whether to use Symfony's ApcClassLoader.
    # Possible values: 0 or 1
    # Defaults to 0 if omitted
    #SetEnv USE_APC_CLASSLOADER 0

    # Prefix used when USE_APC_CLASSLOADER is set to 1.
    # Use a unique prefix in order to prevent cache key conflicts
    # with other applications also using APC.
    # Defaults to "ezpublish" if omitted
    #SetEnv APC_CLASSLOADER_PREFIX "ezpublish"

    # Whether to use debugging.
    # Possible values: 0 or 1
    # Defaults to 0 if omitted, unless ENVIRONMENT is set to: "dev"
    #SetEnv USE_DEBUGGING 0

    # Whether to use Symfony's HTTP Caching.
    # Disable it if you are using an external reverse proxy (e.g. Varnish)
    # Possible values: 0 or 1
    # Defaults to 1 if omitted, unless ENVIRONMENT is set to: "dev"
    #SetEnv USE_HTTP_CACHE 1

    # Defines the proxies to trust.
    # Separate entries by a comma
    # Example: "proxy1.example.com,proxy2.example.org"
    # By default, no trusted proxies are set
    #SetEnv TRUSTED_PROXIES "127.0.0.1"
```
Merge pull request #51 from patrickallaert/EZP-20821

Implemented EZP-20821: Make it possible to configure the front controller without editing it