Ability to interrupt a running deploy

The work has now been down to allow deploys to be interrupted whilst in progress.  This has been done by re-writing the way in which actors were used to run deploys.  Deploy preparation and each individual task is now done separately, allowing a deploy to be stopped between each task if a stop flag is set.

The UI has been enhanced with buttons to stop a deploy in progress.

This also paves the way to run multiple tasks in parallel.