Add DEFAULT_FILE_STORAGE configuration.

Tests in common are run with both CMS and LMS configuration (with different env files). Previously only the LMS configuration file was updated (see https://github.com/edx/edx-platform/blob/master/lms/envs/test.py#L273).

@dan-f and @jzoldak 

Jay, should there be a common test file configuration that both LMS and CMS test.py files can start from?