Expose aws tags and metadata as fleet metadata.

• Access to the aws metadata service. (available from aws instances)
• ec2:DescribeInstances permission, configured for aws cli. (instance profile recommended)

aws-fleet-metadata should be run as a prerequisite to fleet.service. The sample drop-in and service file show how to do this with cloud-config. aws-fleet-metadata outputs to stdout by default, but accepts a flag -o to output to file (general use case, to be used by EnvironmentFile).