Add support for STS Tokens (aws_security_token) to all cloud modules

Extends get_ec2_creds() in lib/ansible/module_utils/ec2.py to include security token.

Modifies all cloud modules to respect security token and provide to service-specific boto connections as required.

These changes enable ansible's cloud modules to use temporary credentials provided as a result of assuming an IAM Role, rather than using IAM user credentials.