Replace Experiment with something else in the code

[kalantar]

We've more or less eliminated the notion of an experiment from our description of iter8. Instead we talk about performance tests. Should we rename the object Experiment as PerformanceTest, PTest, Test, Iter8Test or something similar?

[sriumcp]

Perhaps ok to leave it as Experiment ... for now? This is anyway not user-facing.. if we do change it later (e.g., to Test), it wouldn't constitute a breaking change.