Make it possible to auto map with multiple OMs

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ?
| Fixed tickets | https://github.com/doctrine/DoctrineBundle/issues/60
| License       | MIT
| Doc PR        | -

see also https://github.com/doctrine/DoctrineBundle/pull/322