register doctrine alias mapper pass

 to support CmfContentBundle:StaticContent shortcut

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | travis
| Fixed tickets | https://github.com/symfony-cmf/symfony-cmf/issues/204
| License       | MIT
| Doc PR        | -