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
https://github.com/gds-operations/vcloud-launcher/pull/77/files#diff-afd2fa764f811e5a5ce141138813f77bR41 relies on the command being the entire executable we want to run.

But the point is that by not writing code to do something that isn't part of a current need, I create working software more rapidly. When I do have needs later that contravene this, then I change the code to accommodate them. I'm not trying to create perfect software that satisfies those possible future needs.

To take the discussion to the extreme, this code doesn't give the user a pony, but they might want one - does that mean I should write code that does? :smiley: