add integration test project for REST API

by default the tests will be skipped when running `mvn test`, use -Dgl2.integration.tests to enable them.
the default target is http://localhost:12900 using basic auth credentials of admin:admin

you can override these by using system properties:
  gl2.baseuri
  gl2.port
  gl2.admin_user
  gl2.admin_password

The tests currently expect that all necessary services are running and configured