Simpify assignment of workload parameters target_throughout and search_clients

[tlfeng]

Description

Simplify the assignment of parameters target-throughput and search_clients to reduce the code duplication, which was introduced in the commit dd2ff3b.
Initially I planned to use Jinja macro syntax to reuse the redundant codes, but after looking at the target codes, I think simplifying the code itself might be a better option.

For target_throughput, I propose to change from

{%- if not target_throughput %}
,"target-throughput": 2
{%- elif target_throughput is string and target_throughput.lower() == 'none' %}
{%- else %}
,"target-throughput": {{ target_throughput | tojson }}
{%- endif %}

to

"target-throughput": {{ target_throughput | default(2) }}

For search_clients, I propose to change from

{%- if search_clients is defined and search_clients %}
,"clients": {{ search_clients | tojson}}
{%- endif %}

to

{%- if search_clients is defined %}
"clients": {{ search_clients }}
{%- endif %}

or from

{%- if not search_clients %}
,"clients": 2
{%- elif search_clients is defined and search_clients %}
,"clients": {{ search_clients | tojson}}
{%- endif %}

to

"clients": {{ search_clients | default(2) }}

Some reasons:

1. For the field target-throughput, there was a specified value assigned to everywhere has got the field, so it's typical to use "target-throughput": {{ target_throughput | default(2) }} to assign a default value if adding the ability to assign custom value. This way is commonly used in the repository, such as https://github.com/opensearch-project/opensearch-benchmark-workloads/blob/1/nyc_taxis/test_procedures/default.json#L34 and https://github.com/opensearch-project/opensearch-benchmark-workloads/blob/1/so/test_procedures/default.json#L12, and there is no need to use the current tedious statements for "target-throughput".
2. Validating the data type of the input value is not useful, and there is no such validation for other parameters in this repository.
   Although there is a difference in the error message after the code change, but I think it's acceptable, because there is no such tedious value validation for other fields.
   For example, assigning non-number value to target-throughput after my code change will throw the following error.

opensearch-benchmark execute-test \
--workload-path=/home/ftianli/github/opensearch-benchmark-workloads/nyc_taxis --test-procedure append-no-conflicts --test-mode \
--workload-params="target_throughput:text,search_clients:4"

[ERROR] Cannot execute-test. Error in test execution orchestrator (Could not load '/home/ftianli/github/opensearch-benchmark-workloads/nyc_taxis/workload.json': Expecting value: line 454 column 32 (char 11316). Lines containing the error:

          "operation": "default",
          "warmup-iterations": 50,
          "iterations": 100,
          "target-throughput": text
-------------------------------^ Error is here
          ,"clients": 4
        },

The below is the error before my code change:
I guess the error message comes from opensearch-benchmark validates the user input of "target-throughput", although the field is not assigned in json file.

[ERROR] Cannot execute-test. Error in test execution orchestrator (Task [default] specifies invalid target throughput [text].)

4. Looks like the expression and search_clients in existing statement if search_clients is defined and search_clients make no sense in this case. Empty value such as None or none or "" still be assigned in json file.
5. I don't think the expression | tojson is necessary here in ,"clients": {{ search_clients | tojson}}, looking at the document https://jinja.palletsprojects.com/en/2.11.x/templates/#tojson, tojson() used to escape certain special characters in HTML contexts. The 2 parameters target_throughout and search_clients has number value, and I don't think escaping character is necessary here.
   Besides, using expression | tojson seems comes from Elasticsearch code elastic/rally-tracks@dcd691f, but don't understand the meaning of that.

Testing:

opensearch-benchmark execute-test \
--workload-path=/home/ftianli/github/opensearch-benchmark-workloads/nyc_taxis --test-procedure append-no-conflicts --test-mode \
--workload-params="target_throughput:10,search_clients:4"

opensearch-benchmark execute-test \
--workload-path=/home/ftianli/github/opensearch-benchmark-workloads/noaa --test-procedure append-no-conflicts --test-mode \
--workload-params="target_throughput:10,search_clients:4"

Issues Resolved

#64

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[tlfeng]

I will close the PR for now, although there is no obvious issue with the code change. I will discuss the necessity with the maintainers for this repository.