fs_config: modularize fs_config_generator

This internally structures fs_config_generator.py to be able
to plug in generators to produce different outputs. This
prepares this tool for group and pwd file outputs.

Test: Checked diff and hash of before and after files.
Change-Id: Ie558518ac227dd946d70ab48027698b72a9bc94a
Signed-off-by: William Roberts <william.c.roberts@intel.com>