Prioritize active nodes in URLs list resulting from CM discovery

Currently, Knox's discovery of service URLs from CM is ignorant about the "active"-ness of the URLs it discovers.

For WebHDFS for example, Knox blindly consumes the URLs from CM without any regard for which NameNodes are active. This can result in a stand-by node URL being first in the list in the resulting topology. While Knox will fail-over to the active node on the first request, this fail-over is unnecessary and avoidable.

There are other services for which discovery can be similarly enhanced. Those should be identified as part of this effort, and as much as possible, support for this new behavior should be implemented once for as many services as are affected.