(PUP-2104) Add cfacter setting to enable cfacter implementation.

Adding a --cfacter setting that will replace the facter implementation
with the native facter implementation if the following conditions are
met:

* cfacter is installed
* the version of cfacter is at least 0.2.0 (not yet released).
* facter has not evaluated any facts by the time the setting is set.

To accomplish this last point, certain default setting values needed to
be delay-evaluated as they relied on facts. Therefore a :default
for a setting now accepts a Proc and calls the Proc only once when the
default value is needed.
I was considering this approach for cfacter so that we can reliably change the implementation, so :+1:. One comment to note is that this will immediately disable any ruby facts, which is a related problem.