FEATURE: Provide global replica value for index templates

[jertel]

For users that need to update all of their index templates to use multiple replicas, it's currently a tedious process of updating a large number of settings from 0 to 1. For settings that are typically similar across all index templates, introduce a global setting that will be used, if defined. Or for better flexibility, the global setting will be used unless the individual template's setting has already been customized, in which case that value will be used for that one template.

As there could be other settings, similar to number_of_replicas that could be similar across all index templates, implement in a way that makes it easy to re-use this for future settings.