labs: add snapshot/boot commands to config/scripts.* handler

This patch adds the ability for osbash to understand boot and
snapshot commands in config/scripts.* files.

The syntax for running scripts within the VM remains:

<dircode> <script_name>

For the new commands, it is:
boot
snapshot "Description for snapshot"

The Vagrant-side of the code will simply ignore these commands.

get_script_paths_from_config has been removed and merged into the
execution routine for osbash and Vagrant, respectively.

This changeset puts only the functionality in place. It is going to be
used by a subsequent patch.

Implements: blueprint openstack-training-labs
Change-Id: I2213125715e4cdf4cd2b4f501f3c216e2474b465
Merge "labs: add snapshot/boot commands to config/scripts.* handler"