fix location revision error for multi parent draft

PLAT-332
@adampalay @dmitchell 
Fix problem where a component has multiple parents (of which all other parents except original parent could be orphans) and when user tires to export the course with draft version of that component then "get_parent_location" method raises "AssertionError" for location revision of that component.
Yes, you are right that `get_parent_location` will return `vertical_x1a (draft)`. I will update the test.
In `_get_raw_parent_location` method we add `revision` (None for publish and 'draft' for draft). Since this fix is for draft branch so with query we get results sorted by revision (with 'draft' revision location first) and then we pick first one from that array or results.