Run a script on launch

This is more for review than to be merged speedily.

    In order to trigger actions when a VM is launched, we should be able to
    specify an arbitrary script to run which receives information about the
    VAPP via environment variables and is capable of executing further actions
    (such as notifying other systems that the VM has been created).

I've chosen to implement that by running a script and passing it the entire vapp_definition hash as an environment variable.

I'd like to know:

1. Am I on the right track?
2. How could I test this better? It feels like the test I have added is a little inadequate.

This is for https://www.pivotaltracker.com/story/show/66416966
They are denied because it avoids having to deal with the case of a command with arguments further in the script, because that is simpler.

I am not really clear on the use-case for supplying arguments to be honest - nothing in the code allows those arguments to be parameterised based on current execution so you'd be supplying the same arguments to the script for every VM you were creating. I would expect that the executable would take no arguments and would change behaviour based on the environment variable supplied.

I'm also not convinced I should add code for a need we haven't got. Surely it should be up to someone who *does* have a real need to supply arguments to make those changes later when necessary?