Use wait_for to allow course image time to update

I can't reproduce this failure locally, but it's occurred in Jenkins:

    AssertionError: Was looking for /c4x/MITx/999/asset/image.jpg,
    found http://127.0.0.1:46777/c4x/MITx/999/asset/images_course_image.jpg

This is a possible fix, and a better option than #1242 

@cahrens Does it make sense to merge this first, then #1242 if we see another failure?