Fixes #2850: add hard classes pass_1 pass_2 pass_3 for each iteration over promises

This Pull Request adds a Hard Class "pass_1", "pass_2" or "pass_3", depending on which iteration cf-agent is currently doing across a bundle.

This is in response to a [recent mailing list discussion](https://groups.google.com/forum/#!topic/help-cfengine/xhLRGvVq3eY) and [Redmine ticket #2850](https://cfengine.com/dev/issues/2850).

Thoughts?