Have Http Liveness Probe report "Unhealthy" on connection failure instead of "Unknown"

Based on #3041: modfiy HTTP liveness probe so when it fails due to connection issues, it will return an "Unhealthy" status instead of an "Unknown" status thus forcing kubelet to restart the container.
The reasoning behind this change is that if Kubelet can't establish a connection to the container service, considering they're on the same host,  it is fair to assume that the container is in an unhealthy state, and should be restarted.
Random thought for the future: I wonder if we should use a custom transport with connection caching turned off, instead of just straight http.Get.