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
I don't think it being an MVP has any bearing on this. This pull request explicitly denies the use of command-line arguments; which I think is fine so long as we have a good reason. Why would they be denied?