Adds squashfs overlay support

This stucture is based on tar overlay support for layman.

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.

constants.py: Adds MOUNT_TYPES to constants to keep track of
mountable overlay classes.
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.