Ensure diversity sampling instead of random (proportionate) sampling

[mlopatka]

https://github.com/mozilla/python_mozetl/blob/32d78c34dbb3c9ff5542f1ebc110f5aeb7fce340/mozetl/taar/taar_similarity.py#L131

The diversity of the donor pool is only ensured by the assumption that higher level clustering is substantially diverse. This could be improved by verification of cross-cluster diversity in the addons space.