Additional logic to handle more course_image URL edge cases
This changes logic to allow more missed use cases of course_image to
work properly.  The cases are:

. XML courses with the course_image attribute set
. Mongo courses that are imported without a contentstore
. Mongo courses that have course_image set but don't have a content store

It also exports default images_static_course.jpg to
images/static_course.jpg to handle a use case where a course author
uploaded an image to the default location in studio without using the
studio interface for adding course images, they then export the course, and then
import it without a contentstore
Corrected test names and doc strings
Simple fixes

- Ensure temp files are cleaned up.
- Refactor a function with three return statements.
Merge pull request #1 from edx/ned/tweaks-for-pr-3464

Simple fixes
Merge pull request #3464 from mitocw/cg/course_image_fixes

Additional logic to handle more course_image URL edge cases