Implement control-plane placement templating

[semtexzv]

- What I did
First pass on implementing templating of tasks for placement purposes. When combined with multiple nodes nodes labeled like srv=1, srv=2, allows for pinning of individual instances to nodes like so:

services:
  srv:
    deploy:
      replicas: 2
      placement:
        constraints:
          - node.labels.srv=={{Task.Slot}}

- How I did it
Re-uses the most of the existing templating code, not sure about where to template the task struct though.

- How to test it
Actually no idea. Open to suggestions how to make this production-level code instead of an experiment.

- Description for the changelog
Added support for partial templates in placement constraints.