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        | TBD
hm, i would not expect it to be set. maybe we just leave the initializer tied to sonata, and say people doing their own admin solution have to do their own initializer if their solution needs it. its not that hard to do.