Don't wait for replicas when reopening an index.
Don't wait for replicas when reopening an index.

Waiting for replica shards isn't necessary for reindexing content, and
causes builds to fail in an environment where a non-master node is not
running.

Also, add a load of comments around the `wait_until_ready` method to
explain some of the assumptions around it.

alphagov/rummager,alphagov/rummager,theodi/rummager,theodi/rummager