Provide wrapper to connect to phpBB SaaS MySQL server

Summary:
The phpbb CLI command will allow to manage the different phpBB
available on SaaS.

This change bootstraps the command skeleton, forked from certbot
(see change d03884fb) and implements a first command.

phpbb storage fires a MySQL container linked to the phpbb_db
MySQL server container (the one deployed in change c23ba803749d)
and runs a MySQL client, connected as root.

This allows to easily administrate the MySQL server for the phpBB SaaS.

Ref T1377.

Test Plan:

• shellcheck linter
• ssh -t equatower phpbb storage runs correctly the MySQL client

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1377

Differential Revision: https://devcentral.nasqueron.org/D1518