fs_config: add unit tests

Add some initial unit tests.

The unit tests themselves are inlined into the tool and
can be executed by running the "test" commandlet.

Example:
$ python -m unittest test_fs_config_generator.Tests
.............
----------------------------------------------------------------------
Ran 13 tests in 0.004s

OK

Test: run the test commandlet and observe for failures.

Change-Id: I1bada385fa841fd50fa958997d440f1198e15198
Signed-off-by: William Roberts <william.c.roberts@intel.com>