Prepend PIM TwigBundle configuration

```
Bug fix: no
Feature addition: no
Backwards compatibility break: no
Unit test passes: ?
Behat scenarios passes: ?
Checkstyle issues: no
Documentation PR: 
Fixes the following jira: ~
```

In order to provide our bundles configuration inside the pim-community-standard edition, we have to prepend it whenever they are registered.

## Overriding Issue
For example,
we prepend the following configuration:
``` yaml
twig:
    globals:
        bap:
            layout: '::base.html.twig'
            start_route: 'oro_default'
```

If the user overrides this configuration inside its `app/config/config.yml`,

there is no problem if he defines another key than `bap`:
``` yaml
# app/config/config.yml
twig:
    globals:
        foo: 'bar'
```
result:
``` yaml
twig:
    globals:
        bap:
            layout: '::base.html.twig'
            start_route: 'oro_default'
        foo: 'bar'
```

the whole `bap` key is overrided otherwise:
``` yaml
# app/config/config.yml
twig:
    globals:
        bap:
            layout: 'SomeBundle::base.html.twig'
```
result:
``` yaml
twig:
    globals:
        bap:
            layout: 'SomeBundle::base.html.twig'
            # No more start_route config key
```