Write script to determine is deployment is successful for CI

[jamesgorrie]

Ref #898

Write a script that can compare the deployment:label tag of a service against the tasks running within that service to determine that a service has successfully deployed.

This can then be added to the release to stage step of the build as a "completed" check.

[kenoir]

This should be a weco-deploy command to keep the deployment logic together. The command could then record a deployment as successful in dynamo.

Possibly a deployment should have a list of statuses to accomodate checks run against it (deployment success/200s/browser tests etc).