Run a post-launch-cmd on VAPP launch

See also: #77 - Apparently if I force push I can't reopen a PR which is annoying!

> 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).

This commit adds the -p flag which will run the executable specified on VAPP launch. It will pass the entire vapp_definition hash as the VAPP_DEFINITION environment variable and the script will need to process that for further information.

This is simply just a tidied up commit history which already addressed all the comments in #77 

This is for [#66416966](https://www.pivotaltracker.com/story/show/66416966)