Scripts to clean disk and shut down

This changeset adds two simple scripts: one fills the unused space
of the root disk with zeroes to allow for better compression of the
resulting disk image, the other shuts the computer down.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: I2e80518733b0e1e17a59192c6e2522f423709551
Merge "Scripts to clean disk and shut down"