userportal,webadmin: WelcomeServletTest improvement

This patch applies common pattern for testing servlet classes:

    public class MyServlet extends HttpServlet {

        ...

        private ServletDependency dep;

        @Override
        // Called by servlet container at runtime
        public void init() {
            init(new ServletDependencyImpl());
        }

        // Package-private visibility, method used by MyServletTest
        void init(ServletDependency dep) {
            this.dep = dep;
        }

        ...

    }

In this patch, WelcomeServletTest uses BrandingManager mock instead
of real BrandingManager implementation to ensure WelcomeServlet is
tested properly in isolation, i.e. using mocked dependencies.

Change-Id: I2d654ce7ca57f8bd8962ab668a0c13e1e09d80f3
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>