Add support for AWS_SESSION_TOKEN environment variable.
Add support for AWS_SESSION_TOKEN environment variable.

According to http://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs
the "official" environment variables that should be used for
AWS credentials should be:
AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY and AWS_SESSION_TOKEN
This patch adds support for the latter (the first two are already
supported).

thaim/ansible,thaim/ansible