Run Gerrit.

This role assumes it will have a local MariaDB running adjacent to the
Gerrit service. Variables below configure MariaDB connection details.

**Role Variables**

.. zuul:rolevar:: gerrit_reviewdb_mariadb_dbname
   :default: gerrit

   The database to make and connect to.

.. zuul:rolevar:: gerrit_reviewdb_mariadb_username
   :default: gerrit

   The user to make and connect with.

.. zuul:rolevar:: gerrit_reviewdb_mariadb_password
   :default: <unset>

   The password to set for ``gerrit_reviewdb_mariadb_username``