Introduce the possibility to run unmanaged Arquillian containers in the "lra/test" module

The "lra/test" module has been designed to run integration tests using managed Arquillian containers. In particular, the observer defined in the module "arquillian-extension" does not kick in when an unmanaged Arquillian container is started. This restricts the type of possible tests. To solve this issue, the usage of "DeploymentScenarioGenerator" is suggested (from Arquillian documentation) instead of an observer.

Moreover, the "arquillian-extension" module forces dependent modules to use "ArquillianLRAExtension" as this extension is directly declared in the "arquillian-extension" module. This design needs to be changed to let dependent modules choice which extensions they want to use.