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.
@adampalay I further looked into the `get_parent_location` method and we indeed get the parent location as `draft` from `_get_raw_parent_location` method on `draft` branch. But we return that location `as_published`.
So ``get_parent_location` returns version agnostic location (revision always none).
https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/modulestore/mongo/base.py#L1390