Add WORDPRESS_DB_SSL variable to support connection to the DB with TLS

[jrrdev]

To enforce using TLS to connect to the database, before this PR the only way was to use:

WORDPRESS_CONFIG_EXTRA: |-
        define('MYSQL_CLIENT_FLAGS', MYSQLI_CLIENT_SSL);

Nevertheless, the entry-point was never using TLS to connect to the database.
This leads to connection errors if MySQL was configured with --require-secure-transport=ON and can trigger some fail2ban mechanism.

This PR adds a new env variable WORDPRESS_DB_SSL to configure properly the TLS connection in the entry-point and in Wordpress.

[tianon]

See #557 (which you should be able to more easily test once docker-library/official-images#9390 merges), which completely changes the way this is managed (and is our planned solution going forward).

With that PR's change, you should be able to define WORDPRESS_CONFIG_EXTRA as you have here and it will Just Work. 😄

[tianon]

Closing given that this should "just work" with #557 (which can be tested by using the beta variants of the tags on the published images at https://hub.docker.com/_/wordpress, and will become the default in WordPress 5.7+).