Per environment lamba configuration JSON files

[winglian]

This PR allows shep to read lambda.{env}.json files in the project and function directories. This should allow more flexibility when deploying to VPCs in different environments, as well as using different runtimes (nodejs4.3, nodejs6.10, etc) in a dev environment before upgrading a prod environment.

[reconbot]

Please add some docs explaining this, other than that, this is great!

[reconbot]

When would you call this without specifying the env? If we never call it that way, might as well throw if either of these arguments are missing

[winglian]

@reconbot The only place that might call lambdaConfig without an env parameter would be https://github.com/bustle/shep/blob/master/src/util/load.js#L9. This particular use case is problematic as it is used to get available environments, but requires querying aws for the alias of the deployed function itself. It is probably sufficient to not worry about environment specific configurations for this particular call as long as nobody is throwing any wrenches into the lambda.env.json file such as giving it a new name.

[reconbot]

I think it pays to have two more specific functions instead of an argument. Our code in general is a little overloaded so it's hard to reason when refactoring and that would make it easier. I really appreciate you putting eyes on all that.