Allow setting Verdict::Segmenter::FixedPercentageSegmenter.salt

This way you can change how the segmenter assigns subjects to groups. 

This is very useful for test suites, to make sure that tests are not depending on the the segmentation logic being deterministic.
```
# In test_helper.rb
Verdict::Segmenter::FixedPercentageSegmenter.salt = SecureRandom.uuid
```

@jeromecornet @peterjm @pseudomuto