Configure alternate driver and format for dummyImages

Some imagine drivers like vips do not support the creation of images on the fly which breaks the dummy images.
This change allows to configure an alternate driver that is used for the dummyImages plus a fallbackFormat that is used when the chosen
format could not be rendered.

Thanks to @gradinarufelix for providing the PR and to @rolandschuetz and @kdambekalns for testing:

Sitegeist:
  Kaleidoscope:
    dummyImage:
      overrideImagineDriver: 'Imagick'
      fallbackFormat: 'png'