Use gateway discovery service as a top-level discovery service

A top level discovery service may advertise nothing but other discovery services. Hosts query the top level DS, then recursively query each DS that reveals, and so on until they've enumerated the whole tree and discovered all subsystems those discovery services choose to show them.

A central discovery service in a datacenter would enable all hosts (or IPUs) to be configured the same way for discovery, then find the subsystems with their namespaces by enumerating them all.

A user with many Ceph clusters might need something like this.

The gateway's discovery service (#63) could support this if its configuration could come form a file instead of OMAP (since the top level discovery service isn't part of any Ceph cluster).

See also: centralized discovery controller (#73 and #72).

Start with the gateway discovery service described in issue #63, then read the centralized discovery controller issues #72 and #73. Define how a top-level service would load configuration outside a Ceph cluster and recursively expose delegated discovery services and their subsystems. Done means the required behavior and configuration path are specified well enough to implement and test.

Written by the indexing model from the issue text.