[IZPACK-1094] Use Maven Failsafe Plugin for integration tests

During the build, integration tests are currently handled by the Maven Surefire Plugin in parallel with the Unit tests in the test phase.

This is not convenient and implies in problems maintaining the POMs.

Integration tests need in several cases packaged artifacts from the reactor build, which haven't been available in the test phase. This resulted in occasional build failures, mostly due to a missing samples/izpack/install.xml from izpack-dist-sources for the IzPack integration tests in izpack-test.

The easiest way how to resolve this is using the Maven Failsafe Plugin for the integration tests, which automatically binds to the correct lifecycle phases.

There have been also done some small cleanups in the build.