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.

test/fn-system-tests/system_test.go

@@ -31,7 +31,8 @@ import (
 )
 
 const (
-	LBAddress = "http://127.0.0.1:8081"
+	LBAddress   = "http://127.0.0.1:8081"
+	StatusImage = "fnproject/fn-status-checker:latest"
 )
 
 func LB() (string, error) {
@@ -276,7 +277,10 @@ func SetUpPureRunnerNode(ctx context.Context, nodeNum int) (*server.Server, erro
 	cancelCtx, cancel := context.WithCancel(ctx)
 
 	// now create pure-runner that wraps agent.
-	pureRunner, err := agent.NewPureRunner(cancel, grpcAddr, agent.PureRunnerWithAgent(innerAgent))
+	pureRunner, err := agent.NewPureRunner(cancel, grpcAddr,
+		agent.PureRunnerWithAgent(innerAgent),
+		agent.PureRunnerWithStatusImage(StatusImage),
+	)
 	if err != nil {
 		return nil, err
 	}