Redmine#4019: update DC to use 3.6 features

see https://cfengine.com/dev/issues/4019

* less boilerplate
* data containers used natively
* simpler runfile
* CFEngine::stdlib sketch removed, we assume the regular `promises.cf` structure is used
* CFEngine::dclib lost weight (common stuff moved to stdlib)
* `motd` sketch rewritten to show new syntax
* all sketches need some small work and then testing