Improve mariadb configuration

• Our mariadb_table_open_cache was too low (64 in the defaults and 256 in the playbooks), bumbed to the upstream default (2000).
• The playbooks set innodb_buffer_pool_size to 64M, but the default was bumped to 128M in f164d000 so the playbooks should be likely updated too. Which value should we use? Generally, higher is better (depending on the amount of available RAM): see documentation.
• The playbooks set query_cache_type to 0 (i.e. OFF), but this is not enough to disable the query cache - setting query_cache_size to 0 is necessary! See the warning in the Mariadb documentation. I assume the query cache should be disabled, since it is explicitly deprecated in MySQL and Mariadb has many warnings about it too. Should I disable it in all playbooks?

CC @grazzolini @jelle @svenstaro