Use a default image for frameworks that don't have a checked in thumbnail

I'm looking at https://github.com/Capgemini/mesos-ui/tree/master/src/public/assets and it appears the default broken image thumbnail is rendered when a scheduler not in that list is used. Would it be possible to have a default?

Start by inspecting src/public/assets and the existing scheduler thumbnail rendering path. Add a default image for schedulers without a checked-in thumbnail, then verify that an unlisted scheduler no longer displays the broken-image placeholder.

Written by the indexing model from the issue text.