Add MariaDB server config for optimizatons

[swalkinshaw]

Opening a new PR so I can more easily more this. Co-authored by @paulbrzeski

Original PR description from #1443:

Sharing a couple of changes we've made to our Trellis so we can fine tune MariaDB settings for high traffic websites.

Changes include,

• MariaDB/innodb optimisation
• Small improvements to make things more semantic
• Various configs to support above

This PR only provides the defaults, you can override these settings like any other using your group_vars configs.

Example settings from our production servers below,

MariaDB for a server with 4GB of RAM and sharing environment with PHP and Nginx
New variables to add to group_vars/%ENV%/mariadb.yml,

mariadb_set_innodb_buffer_pool_size: true
mariadb_innodb_buffer_pool_size: 512M
mariadb_set_innodb_log_file_size: true
mariadb_innodb_log_file_size: 128M

More info - https://mariadb.com/kb/en/innodb-system-variables/#innodb_buffer_pool_size

(previously #1430)