Configure sandbox using environment variables

[JayH5]

Currently there is only one way to configure sandbox applications: vxsandbox.resources.config.FileConfigResource. This requires a JSON configuration file.

In order to be more Docker/Mesos friendly, it would be good to be able to configure the application using environment variables, using something like "vxsandbox.resources.config.EnvVarConfigResource".

[hodgestar]

I was wondering whether an environment variable was a sane place to put a whole Javascript file. On my machine the maximum size of argument length is about 2M, which is probably fine for a Javascript app, but could get tricky if something has a big config item (e.g. translations for 11 languages).