Create a mechanism of fact dependency on adding facts

[ricardodani]

The holmes.validators.total_requests:TotalRequestsValidator is actually a Facter, but is dressed as a Validator because he needs to wait for some Facters to be processed to then can process your facts.

Implementing a fact dependency, (inspired on South`s depends_on) can solve this problem.

Fixing this we can make the TotalRequestsValidator a real Facter