Isolate test verticle from testcode

* Moved main project sources into core module without changing the artifactId.
* Moved test verticle into maven module test-module. This module will not be deployed but must be installed for testing purposes.
* Removed bogus files and maven plugins for previous approach of test verticle bundling.
* Test module version was pinned to 1.0.0-SNAPSHOT and should not be changed. Problems with maven release plugin can easily be workarounded using maven profiles.