read config from env vars

this is an alternative implementation to https://github.com/intuit/simple_deploy/pull/235

this basically introduces the `--read-from-env` flag for all commands (with the exception of `environments`). this flag will load 4 aws related env vars to use for the config:

* `AWS_ACCESS_KEY_ID`
* `AWS_SECRET_ACCESS_KEY`
* `AWS_SECURITY_TOKEN`
* `AWS_REGION`

you cannot specify both `--read-from-env` and `--environment`. this includes support for temporary credentials.

*note that this implementation does not support notifications.*

thoughts?
was unsure of this one. the aws cli uses `AWS_DEFAULT_REGION`. i don't think the "default" portion makes sense in this context, but can be convinced to leverage it just to stay consistent.