Fix transcripts in XML course when no static_asset_path is set.

Case where static_asset_path is not set in XML course.

In such case use data_dir.
Issue: https://openedx.atlassian.net/browse/BLD-67

@cpennington @carsongee 
@jarv @cpennington  Moved adding `STATICFILES_DIRS` to devstack settings to this PR: https://github.com/edx/edx-platform/pull/4559