Network function virtualization has enabled data center providers to offer new service provisioning models. Through the use of data center management software (cloud managers), providers allow their tenants to customize their virtual network infrastructure, enabling them to create a network topology that includes network functions (e.g., routers, firewalls), either chosen among the natively supported catalog or provided by third-parties. In order to deploy a ready-to-go service, providers have also to take care of pushing functional configurations into each network function (e.g., IP addresses for routers and policy rules in firewalls). This paper proposes an architecture that extends current cloud management software to enable the configuration of network functions. We propose a model-based approach that exploits the use of additional software components, i.e. translators and gateways, which are network function-agnostic, i.e. they are vendor-neutral and not specific for a particular type of network function, and do not require any change in the network functions. A prototype of this solution has been also implemented and tested, in order to validate our approach and evaluate its effectiveness in the configuration phase.