(PDB-718) Fact path endpoint

This implements a "fact-paths" endpoint that returns list of fact paths/types
according to the specification here:
https://docs.google.com/a/puppetlabs.com/document/d/1vWE6tBmsPmMIYKQWV8Pwes_NIR9lEMTYjUH8zJ_AlMI/edit?pli=1
fact-names is meant to be a clue for auto-complete functionality on the GUI. Having said that, there is a case for all types of side-constraints to be useful for this problem, but they will probably not be used as often as the global view we are showing. For example, if the GUI only wanted global paths, it wouldn't be as performant to show all nodes and all fact-paths would it?

I think if the GUI wanted to constrain its auto-complete to an individual node, it could get the entire factset to do that, or the entire fact-nodes for that node. So we kind of already have an answer for that case.