28 add wsrep user creation helpers

[mikk150]

Adds two new environment variables called MYSQL_WSREP_USER and MYSQL_WSREP_PASSWORD that can be used to authenticate replication slaves

should help with #28 (at least works for me)

[yosifkit]

We appreciate the contribution. We are extremely hesitant when adding new environment variables. If something can be done with the available knobs and tools given, we defer to them. These two could be done within a /docker-entrypoint-initdb.d/ script or sql file without the need to modify the entrypoint.