Added --lvm-auto-snap guessing option

The lvm group and volume name now can be
guessed automatically using the --lvm-auto-snap
option. The corresponding blueprint is available:

- https://blueprints.launchpad.net/freezer/+spec/lvm-snap-auto

Also several pep8 style improvements are implemented.

Change-Id: Ic9fcca4b81b1ec0c78baeeef6be1d5e22355e551