[BugFix] [Feature] Fixes issue #1682

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | #1682
| License       | MIT

Makes the checkout steps configurable, for now only the templates can be configured. Of course, you could also override the default templates by doing so in `app/Resources` or overriding the bundle. But since you are tight to the path `Frontend/Checkout/Step:[name].html.twig` it is not very flexible. So with this PR you could define your own way to where you want your templates to be stored. This is similair to the routing part, where you can also configure your product and taxon templates.

Usage:
```
sylius_core:
    checkout:
        steps:
            security:
                template: CheckoutBundle:Step/security.html.twig
            finalize:
                template: AnotherBundle:Step/finalize.html.twig
```