Allow merged contributions for the repository config

Configuration for repositories is now of the form: <extension target="org.nuxeo.ecm.core.storage.sql.RepositoryService" point="repository"> <repository name="default" factory="org.nuxeo.ecm.core.storage.sql.ra.PoolingRepositoryFactory"> <schema> ... </schema> <indexing> ... </indexing> ... (see http://doc.nuxeo.com/x/hwQz ) </repository> </extension> When several contributions are made to the same repository name, they are merged. The old syntax that contributed to two extension points, org.nuxeo.ecm.core.api.repository.RepositoryManager (repositories) and org.nuxeo.ecm.core.repository.RepositoryService (repository) is still supported for backward compatibility but will emit a WARN message. The two nested <repository> elements on the old extension point org.nuxeo.ecm.core.repository.RepositoryService can be merged, and the label="..." that was previously on the org.nuxeo.ecm.core.api.repository.RepositoryManager extension point can be added as an attribute to the new <repository> element.