Put index template api: unified PUT/POST behaviour in relation to create parameter

The put index template api supports the `create` parameter (defaults to false), which tells whether the template can replace an existing one with same name or not. When invoked through REST using the POST method, the `create` is currently forced to `true`, which is not consistent with our other apis.

Unified its behaviour between PUT and POST method. Also added create parameter to the rest spec (was missing before) and a REST test for create true scenario.