WIP: v3.2.0 - firewall support, fixes, improvements

[nxet]

This PR replaces #60, adding more improvements and support for firewall configuration.

• [fix] add mariadb_port default value in defaults/main.yml, commit 9a9a44d
• [fix] list dependencies in README (probably only useful for users which only installed ansible-core), commit 9a9a44d
• [fix] actually use mariadb_mirror config option (see WIP: [FIX] support for MariaDB mirror URL config option mariadb_mirror #60 for more info), commit f181dd1
• [fix] use static import_tasks instead of dynamic include_ (to propagate tags to all children tasks), commit 9896090
• [fix] updated all tasks to use FQCNs instead of base role/module names, commit d1b9834
• [feat] firewall tasks to open ports, commit f20c402
• [feat] detailed tags to all tasks, commit 9689311

The changelog has been updated with all notable changes, alongside with a new version header (using placeholder date of today).

I would really appreciate the commit history being preserved, instead of squashing on merge.
And of course I'm looking forward to discuss in detail any of my proposed changes if required.