[DependencyInjection] Non-default XML namespaces not supported

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The DIC is not able to parse XML configs where the container configuration is not the default namespaces. Such configurations are used for example, in the [DoctrineBundle test suite](https://github.com/doctrine/DoctrineBundle/tree/master/Tests/DependencyInjection/Fixtures/config/xml).

This PR adds a failing unittest for that case.

I have tried to fix the issue as well, but I didn't find a good way of fixing it without rewriting the entire XmlFileLoader using DOM instead of SimpleXML. Hence I am just submitting the test case, in the hope that someone smarter than me can fix this issue.