FixturenatorDefinition with a callable valueGenerator should be lazy-eval'd

[apinstein]

Right now if you pass a lambda to a definition it's eval'd only once, which means that each "new" instance generated will share the same data from the original execution time.

This should be refactored to lazy-evaluate.

Also, we should pass in not only the "new" object, but also the overrideData (from resolveData).