First, you should always use:
http://keycloak.github.io/docs/userguide/html/index.html for the latest documentation.
Second, you don't have to change anything in keycloak-server.json file for MySQL setup.
What you do is modify the KEYCLOAK_HOME/standalone/configuration/standalone.xml file - which is a Wildfly configuration file, and change the KeycloakDS definition there to use MySQL rather than H2.
In order to install mysql driver into Wildfly you have to create a new module for it under KEYCLOAK_HOME/modules directory, and download a .jar file.
I think it goes through all the steps ...