tracing provenance of hard classes for cf-agent

Description:

(This was suggested by Nick Anderson and was recently asked on help-cfengine)

It's currently easy to test for @net_x_y_z@ to see if the subnet is @x.y.z@. But there's no easy way to ask "what subnet did cf-agent detect, that ended up in the @net_x_y_z@ class."

The core problem is that we can't trace the provenance of the "subnet" piece of knowledge, and we don't know that it determined the subnet to be @x.y.z@.

The solution is in my opinion surprisingly simple. In addition to defining the class @net_x_y_z@, the agent should also fill in a special array:

@sys.knowledge_trace[subnet] string => "net_x_y_z"@

This is cheap and easy, and should not be hard to implement.

Feature use case:

• map hard classes to variables (often asked on help-cfengine)
• provide a full list of the hard classes currently defined
• structure the system knowledge better

Current planned features:

• implement the array suggested above

Possible enhancements:

• none

Issues:

• naming of array keys has to be systematic and not ad-hoc

Cross feature mapping:

• none

Demoed for externals:

• none

Documentation:

• manual update