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

Example of VirtualHost additional config
```Apache
# 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 1

# 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 0

# Defines the proxies to trust. (See Symfony\Component\HttpFoundation\Request::setTrustedProxies())
# If your application is behind one or more reverse proxies you manage and trust, set them here.
# 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"
```