Add support for using unsquashfs to uncompress the microstack snap
followed by 'snap try ./squashfs-root/'. This enables installation
of the snap as an rw mount, and local files can be modified in
./squashfs-root/ and will go live instantly. See 'snap try --help'
for more details.
New tox targets are added for snap-try, snap-try-basic, and
snap-try-cluster.
Change-Id: I54fb8dc864fd4f346f20ae986155ad36bb7c1fac
c853b3e8c8