Set a Maven Property indicating if a database reused or created.

This feature can be useful to control the behaviour of subsequent Maven steps. For example, if a database is filled with default data, a user might want to skip this step if an existing database was reused. See “example-database-reuse.apt.vm” for an example.
Defensive coding as this is a runtime issue.

I don't expect this feature to be widely used, so I don't want to break everybody else in case it doesn't work. Logging a warning seemed more appropriate to me at the time.

Specifically, I am not sure how well supported the 'defaultValue = "${project}"' is on older Maven versions. There is also some internal discussion within the Maven team about the future of "${project}". I added unit tests, so should it change on future version, we hopefully notice and can adapt early. But people just using the plugin shouldn't have to worry about that, or should they?