Remove default index shards/replicas counts

Fixes https://github.com/sksamuel/elastic4s/issues/219.

I have a couple of questions/comments about this - see them in the diff.
A lot of the tests _explicitly_ set `shards` in their index creation, even thought they aren't really concerned with it (e.g. in this case it's about setting mappings). So it might be nicer to just remove that explicit `shards` call completely. What do you think?