Updated ansible uri module HEADERS_ field

This fixes a deprecation warning that gets displayed when running
the kibana/post_config 'Get kibana default indexes' task.

HEADERS_ has been deprecated since ansible 2.1 and will be
removed in 2.9.

https://docs.ansible.com/ansible/latest/modules/uri_module.html

TrivialFix

Change-Id: I177113c606119505c6cb69c66a326f7cbdaf2196