Add DEFAULT_FILE_STORAGE configuration.

Tests in common are run with both CMS and LMS configuration (with different env files). Ensure the DEFAULT_FILE_STORAGE variables are in both environments.
Return the filename as returned from default storage's save.

It might be different from the requested filename.
Merge pull request #6215 from edx/christina/test-file-cleanup

Add DEFAULT_FILE_STORAGE configuration.
Adding yaml_config, a new settings module

This reads configuration directly from yaml
and imports it directly into settings
Merge pull request #6188 from edx/jarv/config-refactor-wip

moving defaults for yaml config out of common.py