Inject serverless_operator in track templates

[gbanasiak]

Logical continuation of https://github.com/elastic/rally/pull/1750 with additional global variable serverless_operator. The variable holds true if Rally detects serverless cluster and operator privileges, and false otherwise.

I've extended unit tests to cover the addition. For manual tests I've used custom track definition with the following bits:

# track json
{% import "rally.helpers" as rally with context %}
{
  "version": 2,
  [..]
  "indices": [
    {
      "name": "test-index",
      "body": "test-index.json"
    }
  ],
[..]

# test-index.json
{
    "mappings": {
      [..]
    },
    "settings": {
        "index": {
{% if build_flavor != "serverless" or serverless_operator %}
            "number_of_replicas": "{{number_of_replicas | default(0)}}",
            "number_of_shards": "{{number_of_shards | default(2)}}"
{% endif %}
        }
    }
}