kie-server: tests - simulate real-world usage of Maven repos

 * basically two different repos are used. The local server maven repo,
   which should be always available on the machine the server is running.
   Second one is the `remote-repo` which simulates remote respository
   where the kjars are being pushed, e.g. from CI or kie-wb. Test classes
   are deploying to this remote repo and the kie-server is using custom
   settings.xml which point to this remote repo. This way we could also
   use real remote repo in case needed, as we just need to overwite the
   property that specifies the URL
kie-server: add facility to build kjars for testing; add tests with stateful session

 * the kajrs testing are supposed to be stored in src/test/filtered-resources/kjars-sources
   (or src/test/resources/kjars-sources). They should typically use the kie-maven-plugin
   and from the test one can call the Maven and let it build, install and deploy the kjar.
   Example is in StatefulSessionUsageIntegrationTest class.
kie-server: add container isolation test

 * the test makes sure that different containers
   are using different class loaders and thus
   they can operate on different classes with same FQN