Allow to create MySQL database

Summary:
How can we create a database for a new phpBB instance?

The wrapper script runs on the Docker Engine host
and can interact with the SQLite database there,
but not with the MySQL server, as it's in a container.

The wrapper script will so run a container with this
mainteannce script to create the database.

The wrapper part is implemented at D1524.

Test Plan: saas-phpbb sites:createdb foo

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1573