Default AWS_CONFIG_FILE to ~/.aws/config

These changes add a default value of `~/.aws/config` for `AWS_CONFIG_FILE`.  I've also added some small refactors to the config related test cases as well.

Note: I also had to update aws/aws-cli as well because it passes it's own `EnvironmentVariables` to the session object.  I can either update aws/aws-cli with a similar update to `EnvironmentVariables` or I can just remove the `config_file` line from aws/aws-cli and it will pick up the values from botocore.

cc @toastdriven @garnaat