[GH-342] Fix identifier generator strategy for composite identifier

This issue is created automatically through a Github pull request on behalf of jeanmonod:

Url: https://github.com/doctrine/doctrine2/pull/342

Message:

When using the ConvertMappingCommand on a schema that contain a table with a composite key, we got the exception:

Single id is not allowed on composite primary key in entity CollectionFields

This can be fix by setting the identifier strategy to NONE for composite identifier fields