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.
archive.py: Adds check to unmount overlays if they are being
synced.
Adds Squashfs overlay support

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