Have annotation values override system properties in Docker-based tests

The current Docker test framework allows to define the Servlet Engine or the Browser to use in two different ways:
1. by specifying it in the annotation @UITest
2. by giving it in the command line using this kind of argument: -Dxwiki.test.ui.servletEngine=tomcat

However, currently, the annotation value has always the priority over the given parameter. Then the only way to allow passing a value in the command line, is by not setting the value in UITest.

It ultimately means that we cannot change the default value of the Servlet Engine for the test AND to allow using another version by command line.
In order to fix that I propose that the command line parameter value to have always the priority over the UITest parameter value.