change clustering termination contition in taar/taar_similarity.py

[mlopatka]

https://github.com/mozilla/python_mozetl/blob/05d7c1e1e3b0f4b3ea0c2d26f2f4d1f111bae478/mozetl/taar/taar_similarity.py#L97

Currently relies on a hard-coded threshold expressed as number of clusters. Ecological validity of the model would be ameliorated by making use of a condition based on minimal number of members in a cluster.