Support alternate AWS profiles

[raymondbutcher]

We have a case where a customer has a different naming convention for their AWS profiles, so this configuration will work for them but not us:

config "production" {
  aws {
    profile_name = "production"
  }
  ...
}

This PR allows setting the profile name with an environment variable (we'll use direnv) and if it's not set then it will fallback to the other profile name.

The configuration file will look something like this:

config "production" {
  aws {
    profile_name = ["${env.PRODUCTION_AWS_PROFILE}", "production"]
  }
  ...
}

If both users have direnv, then we'll need to put some logic into the .envrc file to conditionally set this PRODUCTION_AWS_PROFILE environment variable. One option is to grep ~/.aws/config for the profile name that we want to use.