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?
I'm with @dan-f - I don't know how this is working in lms/envs/test?
```
>>> TEST_ROOT = 'foo'
>>> PATH = TEST_ROOT / 'bar'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for /: 'str' and 'str'
```