[MariaDB Packagers] MariaDB 10.3.15 release prep

Prep work for the MariaDB 10.3.15 release has begun. Expected release date is Tue, 14 May 2019. Draft release notes and changelog: - https://mariadb.com/kb/en/mdb-10315-rn/ - https://mariadb.com/kb/en/mdb-10315-cl/ As usual, the release notes and changelog are still in draft form at this time and will be updated prior to release. Thanks. -- Daniel Bartholomew, MariaDB Release Manager MariaDB | https://mariadb.com