Allow to set Elasticsearch number of shards and replicas
In the current latest version of Elasticsearch, the number of shards
and replicas are set to 5 and 1 respectively if not explicitly set.
From version 7, the default number of shards will be changed to 1, and
to continue using 5 it must be explicitly set when creating the index.
Explicitly set the values when creating the index, but allow them to be
changed in the gerrit.config. Default the new configurations to 5 and 1
to be consistent with the current defaults.
Bug: Issue 9768