feat(shepherd): Add auto update shepherd role

Deprecates diun as it provides a simpler implementation for docker
swarm. Mark any containers you want auto updated with
`shepherd.autoupdate=true` and the rest with
`shepherd.autoupdate=false`. Everything untagged will not be watched (by
default), though this can be changed by setting the ansible default
variable `shepherd_filter_services: `.

roles/shepherd/meta/main.yml

@@ -0,0 +1,10 @@
+---
+galaxy_info:
+  author: Marty Oehme
+  description: Apply docker swarm container updates
+  license: GPL-3.0-only
+  min_ansible_version: "2.9"
+  galaxy_tags: []
+
+dependencies:
+  - docker-swarm