squashfs.py: Adds proper docstring

config.py: Adds squashfs_command config variable and alters the
name of clean_tar to clean_archive to be used with both archive
overlay types.

tar.py: Changes clean_tar to clean_archive.
Adds squashfs overlay external tests
Centralizes common archive overlay code

This commit brings archive overlay code to one common place, in
an attempt to make maintaining and adding archive overlay types
to layman much easier.

tar.py: Removes doctest tests from tar.py as there is already an
existing external.py test.