Ensure provider suitability is always lazily evaluated

Puppet::Provider::Service::Upstart had a confine added that is run when the provider is loaded.

  confine :true => begin

    initctl('version', '--quiet')

    true

  rescue

    false

  end

This causes issues with running tests under things like JRuby, and also means that it is not possible for the provider to become suitable during the course of a transaction.  Instead of using a begin/rescue/end block which will be evaluated during the provider definition, this should be something that will be evaluated only when checking if the provider is suitable.
 

This probably means creating a new type of Puppet::Confine that is capable of taking a block to be lazily evaluated as the only existing one that does any sort of "call this code later" goes through the feature machinery. We could also modify the existing true and false confines to optionally accept a block.