Allow to reuse an existing database if desired.

"start" is a public method, not a constructor.

Instance is constructed in the AbstractMysqlMojo, but "start" is called either in the RunMojo or StartMojo through AbstractMysqlMojo.execute().

Looking at how this is used, all the necessary properties could be moved into the Instance constructor. It would make the Instance.java a bit longer though (find bugs policies - qulice).