roles/mariadb: Disable the query cache by default

It seems that the usefulness of the query cache is diminishing in
recent years. If your cache is large then the time taken to scan
the cache can be longer than the SQL query itself.

See: https://haydenjames.io/mysql-query-cache-size-performance/

roles/mariadb/defaults/main.yml

@@ -23,4 +23,8 @@ mariadb_login_unix_socket: /var/run/mysqld/mysqld.sock
 # default is 100 but the max I've seen used is 5, so let's reduce it
 max_connections: 33
 
+# disable the query cache by default
+query_cache_size: 0
+query_cache_type: 0
+
 # vim: set ts=2 sw=2: