Add user agent string for AWS provider

[carolynvs]

#105 adds support for setting the user agent string used by the terraform azure provider to include info about porter. We should do the same for any other interested providers. Since I've seen the aws mixin uses an environment variable to set the user agent string too (getporter/aws-mixin#34), let's start there and populate the aws user agent environment variable like we do with the azure one.