Read token from AWS_SECURITY_TOKEN env variable

We have an application that is using temporary security credentials which are not delivered via an EC2 Role.  This PR adds support for setting the Token via the standard AWS_SECURITY_TOKEN env variable.  It does not affect any existing functionality.

More info: http://docs.aws.amazon.com/STS/latest/UsingSTS/using-temp-creds.html#using-temp-creds-sdk-ec2-instances

@mitchellh