Stabilize soapui maven plugin tester

Current tests of the soapui-maven-plugin rely on an external service (http://www.webservicex.net/). In the past weeks, the service has been down quite often so the build failed whereas the plugin works well. So tests have sometimes been disabled (for instance, see commit [4da115827af5fec24fc59be658af44a9e227147b](https://github.com/SmartBear/soapui/commit/4da115827af5fec24fc59be658af44a9e227147b), Disabling plugin tester temporarily since service provider is down).

The purpose of maven plugin tests is not to check that soapui is able to reach external services but that we can run tests as with the GUI.
With this PR, tests do not need any network access. Instead, a webservice is runned on the local instance with the mock goal of the plugin.
In addition, we now also check that the mock goal is working ;-)

I created a new soapui project which contains the mock service configuration and the tests.

The port of the mock server can be configured by using the following parameter when running maven ```-DmockServerPort=12345```

Instead of having a test module for the soapui-maven-plugin, we could use the maven-invoker-plugin directly in the soapui-maven-plugin module as it is done in [maven-war-plugin](http://svn.apache.org/viewvc/maven/plugins/tags/maven-war-plugin-2.3/pom.xml?revision=1391314&view=markup) or [maven-install-plugin](http://svn.apache.org/viewvc/maven/plugins/tags/maven-install-plugin-2.4/pom.xml?revision=1380335&view=markup) (associated tests run to a dedicated profile)