Bootstrap script should be a separate file for compatability with other executables

[heyakyra]

https://github.com/twisted/ampoule/blob/master/ampoule/main.py#L107

The bootsrap script is a string that gets passed as a command to sys.executable with the -c option which presumes that this is being run by python.

https://github.com/twisted/ampoule/blob/master/ampoule/main.py#L282

As a script file is more generic and likely to be accepted as the main argument to whatever executable is running, that should be used instead.