Software-defined Networking (SDN) and one of its most known realization, namely OpenFlow, enabled wide-spread and vendor-neutral programmability of the control plane of modern network equipment. However, despite research and standardization efforts, the management plane is still eluding device-neutral and vendor-neutral programmability. Thus, innovation in the management plane is hampered by a dependency on human experts, domain knowledge that is hidden in human-centered manuals, and the huge amount of diverse device capabilities and configuration interfaces. Recent proposals for vendor-neutral data-models, such as OFCONF, are still lacking majority and do not provide a standardized way of device capability discovery and device status monitoring. Accordingly, we present an architecture that provides a unified interface to the management plane of heterogeneous devices, i.e., SDN and legacy devices. We discuss the properties of the chosen level of configuration abstraction and show how applications northbound of the abstraction layer are well prepared against undesired side-effects of management actions. By example of a popular approach for enabling Open-Flow in mixed-SDN/legacy networks, i.e., Panopticon, we provide a proof-of-concept implementation of the proposed architecture in a real test-bed. We show how a management application can use the abstraction layer to discover and configure QoS options in the network, monitor the devices, and prevent undesired traffic interruptions in the legacy domain, while being operated in parallel with an SDN controller.