I understand that you have multiple clusters, each one with apache ranger installed and you want to have a single database engine for all of them.
You can have a single database engine outside of your HDP nodes, but you need to create a different database name for each cluster. I recommend you also to have a different database user account to administrate each database.
Each ambari and ranger node will need to have access to MySQL host using the port 3306. By the way MariaDB is also supported.
The following link may help you, check the point number 5 : https://docs.hortonworks.com/HDPDocuments/HDP2/HDP-2.6.5/bk_security/content/ranger_admin_settings.h...