Allow to reuse an existing database if desired.

@yegor256 

Is the current implementation with the non-final property acceptable?

If you want to have it set in a constructor the Instance.java would need to be refactored to set properties at that point. 

I also have two more changes currently stashed I would like to commit. One is the -defaults-file you asked me to submit with a different pull request. The second is to set a Maven Variable indicating if the plugin reused an existing database. (Very handy to skip post-processing steps such as filling the database with default values.)