Enable gateway discovery service to act as a centralized discovery controller

[sdpeters]

An NVMe centralized discovery controller (CDC) serves the same purpose as a top-level discovery server (#71), but improves on that by eliminating the need for every host to completely enumerate all the discovery controllers on the network to find the namespaces they can/must attach to.

To be used with many Ceph clusters would require #72.

This feature is just for the ability of the gateway discovery service to act as a CDC. As much as anything this ticket is to distinguish CDC functionality from #71

Centralized Discovery Controllers are defined in TP8010a. See the ZIP file of ratified NVMe 2.0 TPs on nvmexpress.org for the text of TP8010a.