Add possibility to disable a specific namespace.

Hi there, I have added a simple functionality to your excellent bundle: being able to disable a selected namespace. This is particularly useful for dev/test environments where we don't want an actual namespace to be used. I take example of PrestaSitemapBundle which uses this bundle for internal cache. Adding a simple 'disable' option does the trick for not creating a service on compiler pass.

config.yml
```
liip_doctrine_cache:
    namespaces:
        presta_sitemap:
            type: file_system
```
config_dev.yml
```
liip_doctrine_cache:
    namespaces:
        presta_sitemap:
            disabled: true
```