AWS_* env var overrides

[wrightp]

I think we should do this https://github.com/opscode/chef-provisioning-fog/blob/master/lib/chef/provisioning/fog_driver/providers/aws.rb#L175

This will make testing in Travis easier to boot!

[tyler-ball]

Implement the same order of operations from the fog driver ^^^ in the aws driver. There should be feature parity across drivers, even if the ENV key name is different.

[randomcamel]

To consolidate this, here's the comment from the Fog file:

# Order of operations:
# compute_options[:aws_access_key_id] / compute_options[:aws_secret_access_key] / compute_options[:aws_security_token] / compute_options[:region]
# compute_options[:aws_profile]
# ENV['AWS_ACCESS_KEY_ID'] / ENV['AWS_SECRET_ACCESS_KEY'] / ENV['AWS_SECURITY_TOKEN'] / ENV['AWS_DEFAULT_REGION']
# ENV['AWS_PROFILE']
# ENV['DEFAULT_PROFILE']
# 'default'