Allow appName parameter to be optional

[TIBCOrkrajews]

I was going to use this mojo in a build, but one annoying bit of friction is that our app name changes with minor (m.n) releases because it contains the version as part of its name. Yes, I could derive a property for the app name, but that's already specified somewhere else (the .product file) , and I don't know if Tycho exposes the app name from the .product as a property that could be used for the appName parameter.

I suspect that in a lot of cases, users build only one app in a project, so being able to leave out the appName should be OK as long as there is an app of the given productId around would be really useful.