* Adds support for health-checking loadbalancer servers. If a
  health-check fails when dialing, all existing connections to the
  server will be closed.
* Wires up a remotedialer tunnel connectivity check as the health check
  for supervisor/apiserver connections.
* Wires up a simple ping request to the supervisor port as the health
  check for etcd connections.
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>
(cherry picked from commit c51d7bfb)