fn: runner status and docker load images (#1116)

* fn: runner status and docker load images

Introducing a function run for pure runner Status
calls. Previously, Status gRPC calls returned active
inflight request counts with the purpose of a simple
health checker. However this is not sufficient since
it does not show if agent or docker is healthy. With
this change, if pure runner is configured with a status
image, that image is executed through docker. The
call uses zero memory/cpu/tmpsize settings to ensure
resource tracker does not block it.

However, operators might not always have a docker
repository accessible/available for status image. Or
operators might not want the status to go over the
network. To allow such cases, and in general possibly
caching docker images, added a new environment variable
FN_DOCKER_LOAD_FILE. If this is set, fn-agent during
startup will load these images that were previously
saved with 'docker save' into docker.

api/models/error.go

@@ -109,6 +109,10 @@ var (
 		code:  http.StatusBadRequest,
 		error: errors.New("Missing route Image"),
 	}
+	ErrRoutesInvalidImage = err{
+		code:  http.StatusBadRequest,
+		error: errors.New("Invalid route Image"),
+	}
 	ErrRoutesMissingName = err{
 		code:  http.StatusBadRequest,
 		error: errors.New("Missing route Name"),