Allow mongo relation templates to be overwritten by config (2.1 branch)

Every list template can be set in the configuration except for the mongo relation templates. This PR fixes this.

It would also be OK to merge commit https://github.com/sonata-project/SonataDoctrineMongoDBAdminBundle/commit/9f0805cc13a36f834708553694eec0c4dc5e6d50 to the 2.1 branch.