Create-Workload Improvements: Add Default Index Settings for Index.json

[IanHoang]

When OSB creates a custom workload, the index.json only includes mappings from the referenced index and does not include default settings for index settings.

Each index.json should include default index settings. Example:

  "settings": {
    "index.number_of_shards": {{number_of_shards | default(1)}},
    "index.number_of_replicas": {{number_of_replicas | default(0)}},
    "index.queries.cache.enabled": {{query_cache_enabled | default(false) | tojson}},
    "index.requests.cache.enable": {{requests_cache_enabled | default(false) | tojson}}
  },

[IanHoang]

Added it to this RFC https://github.com/opensearch-project/opensearch-benchmark/issues/395