Added ElasticsearchIntegrationTest#ensureSearchable() to prepare indices to be searchable

Currently we sometimes see test failures that fail because not all replicase are
`searchable` which means they are not started yet or still recovering. Yet, the usual
situation is where two nodes have the same clusterstate but the one that acts as
the search target has not yet processed that clusterstate. The requester sees the
shard as started but it's not mark as such on the target node. For now the #ensureSearchable()
just delegates to #ensureYellow() to make sure the cluster is not red. In the future if we have
the possibilty to recover from situations like this in the search logic we can easily test
this by making the impl a no-op. Note: this problem only occurs if you have low number of docs
and the indexing is really quick such that first request are exectued but shards are not
fully `started`