labs: new cmd syntax for config/scripts.*

This changeset introduces a new syntax for commands in the
config/scripts.* files.

Command lines start with "cmd " followed by a command (such as boot or
snapshot) and its arguments (if any).

This allows us to easily recognize command lines anywhere without having
to keep a complete command list.

Implements: blueprint openstack-training-labs
Change-Id: I7a52476f5b9ba24c2746667b5a0b23b4b73d2e87