Workflow API: allow dangling nodes

[josephjclark]

The Workflow API right now won't support dangling nodes - ie, a job node with no edges connecting to it.

This is not a real constraint in the system: the workflow diagram, the database and the runtime all support dangling nodes. Its not a problem, the node simply won't be executed.

There is no need for the Workflow API to be strict about this. It's fine at the moment for our requirements, but when migrating the workflow diagram on to the Workflow API, we'll surely need to relax this constraint.

I actually think we should generally be wiling to save invalid workflows to the database, because they may represent work-in-progress. Imagine not being able to commit to git if your linter fails (I've actually seen this in a real dev shop, it's the worst). It's up to the Workflow Diagram and the runtime (and maybe the provisioning API - although it shouldn't care too much about logical consistency) to handle any strange cases like dangling nodes.