document how AZ selection will work for AWS
settings.provider.az => microbosh resources.cloud_properties.availability_zone
include spec asset for AZ test
Ensure FileUtils available

Reported as unavaiable https://github.com/StarkAndWayne/bosh-bootstrap/issues/194#issuecomment-18213020
Merge pull request #159 from drnic/az

Invalid availability zone: us-east-1b