Changing default replica count in OpenSearch

[gbbafna]

Is your feature request related to a problem? Please describe.

Customers as per their deployment would want to tweak default replica count which is 1 today. Today there is no mechanism to change that .

Though customers can use index templates to specify replica count and wouldn't need to specify same every time customer creates an index .

Describe the solution you'd like

Mechanism to set default replica count via dynamic cluster setting.

Current mechanisms

1. customers specify replica count in every creation request
2. use of index templates .

Motivation

This will be useful when awareness replica balance #3461 is enabled. New indexes without specifying anything in index templates would come up with correct replica count.