make initializer optional

Add a config option to make the initializer optional.
Currently this option is only relevant if sonata admin is used (BC).

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony-cmf/RoutingBundle/issues/250
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/issues/525