omdb: Use qorb for connecting to control plane services?

[jgallagher]

By default, omdb will look up control plane services in internal DNS and pick one. If we're in a sad situation where some instances of Nexus / CRDB aren't accessible, we have to manually point omdb at one of the functional services. In the case where the user hasn't specified a specific address to use and we're relying on DNS, omdb should set up a qorb pool instead, which should do the work for us of picking one of the reachable / functioning services.