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.

config.py: Alters the clean_tar option to clean_archive.

tar.py: Removes doctest tests from tar.py as there is already an
existing external.py test.
Migrates run_command to utily.py
Adds layman-mounter utility script

api.py: Adds Mounter() initialization in the LaymanAPI.
constants.py: Adds MOUNT_TYPES to constants to keep track of
mountable overlay classes.
Adds squashfs overlay external tests
Adds Squashfs overlay support

constants.py: Adds Squashfs overlay type to MOUNT_TYPES constants.