Delay evaluation of guard_interpreter

We currently evaluate guard_interpreter on the parent resource immediately when evaluating a conditional during compilation. This is an issue if you happen to put your guard_interpreter after your conditional in your recipe. But, we don't actual execute the conditional until later during compilation Chef::Resource #run_action -> #should_skip? -> Chef::Resource::Conditional#continue?. This moves the evaluation of the guard_interpreter attribute to #continue?.

Fixes #1943
Oh, I see that we use it in `#configure`, but now we're checking for the presence of a command or block here in `#initialize`, so is it possible for both `@command` and `@block` to be nil in this class?