Add main script: osbash.sh

This changeset introduces osbash.sh, the front-end script. It is little
more than a wrapper around the libraries in lib.

It sources configuration files and libraries, parses options and
arguments, and calls library functions to get the work done.

If a VM cluster was built (rather than just a basedisk), it can
export an OVA package containing the cluster.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: I2136f7bc9bd67d0d11e2a7a9519ac202ec1ad92f
Merge "Add main script: osbash.sh"