Maven plugin should come with a default value for windupVersion

Right now the user of the maven plugin is forced to set a windupVersion which is a bit odd.

I do like the idea of decoupling the maven plugin version from the rhamt version - the PMD maven plugin does a similar thing. They default to a PMD version, which can be overridden assuming it's compatible.

I would suggest to default the windupVersion to the plugin version, which makes sense, since right now the release of the maven plugin is tied to a rhamt release anyways.

The documentation should cover the default and that overriding this value is possible.