Need node affinity setting

When running several workers on different nodes, we need to be able to apply key/value properties on nodes that we can set a workflow's constraint to. We also need a way to put a constraint on workflows to make sure two or more with constraint value of X don't execute on same node.

Ex 1. Node 1-4 have much faster CPUs, so we set property on the nodes of `SPEED=High`. Workflow X that needs to always run only on one of those four nodes should have a node affinity rule placed on it where only a node with property `SPEED` is set to `HIGH`.

Ex 2. Workflow A, B, and C have activities that are disk or memory intensive and never should run at the same time on the same node.

If no node is available, the workflow can be configured to sit in queue for next available or to cancel/not run.