MySQL container configuration

[iNikem]

Currently there is no way to configure the username/password/database name of the MySQLContainer. And one cannot even do it using withCreateContainerCmdModifier because e.g. org.testcontainers.containers.MySQLContainer#getUsername returns hard-coded value

[kiview]

See #400 for a possible solution to this problem.

[iNikem]

Fixed by #400