Add baseRoute*-parameters to new admin classes

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

With the latest `1.0.0-RC1` I see an exception

    FileLoaderLoadException: Cannot import resource ".../app/config/." from ".../app/config/routing.yml". (Please define a default `baseRouteName` value for the admin class `Symfony\Cmf\Bundle\BlockBundle\Admin\Imagine\SlideshowBlockAdmin`)

I am a little bit confused, that this happens, because I thought SonataAdmin would set appropiate values for `$baseRouteName` and `$baseRoutePattern` itself, when they are missing. Unfortunately it seems, that this issue makes the BlockBundle unusable out of the box, when `LiipImagineBundle` exists [1]

However, I realized, that the other `Admin`-class in the BlockBundle has set this properties too and this PR add them for the both new `Admin`-class `SlideshowBlockAdmin` and `ImagineBlockBundle`.


[1] https://github.com/symfony-cmf/BlockBundle/blob/master/DependencyInjection/CmfBlockExtension.php#L52