Refactor CLI initialization (#562)

This cleans up the CLI code significantly.
Also makes some of this re-usable for providers who want to do so.

This also removes the main.go from the top of the tree of the repro,
instead moving it into cmd/virtual-kubelet.
This allows us to better utilize the package namespace (and e.g. mv the
`vkubelet` package to the top of the tree).

trace/opencensus/ocagent.go

@@ -0,0 +1,37 @@
+// +build !no_ocagent_exporter
+
+package opencensus
+
+import (
+	"os"
+
+	"contrib.go.opencensus.io/exporter/ocagent"
+	"github.com/cpuguy83/strongerrors"
+	"github.com/pkg/errors"
+	"go.opencensus.io/trace"
+)
+
+func init() {
+	RegisterTracingExporter("ocagent", NewOCAgentExporter)
+}
+
+// NewOCAgentExporter creates a new opencensus tracing exporter using the opencensus agent forwarder.
+func NewOCAgentExporter(opts TracingExporterOptions) (trace.Exporter, error) {
+	agentOpts := append([]ocagent.ExporterOption{}, ocagent.WithServiceName(opts.ServiceName))
+
+	if endpoint := os.Getenv("OCAGENT_ENDPOINT"); endpoint != "" {
+		agentOpts = append(agentOpts, ocagent.WithAddress(endpoint))
+	} else {
+		return nil, strongerrors.InvalidArgument(errors.New("must set endpoint address in OCAGENT_ENDPOINT"))
+	}
+
+	switch os.Getenv("OCAGENT_INSECURE") {
+	case "0", "no", "n", "off", "":
+	case "1", "yes", "y", "on":
+		agentOpts = append(agentOpts, ocagent.WithInsecure())
+	default:
+		return nil, strongerrors.InvalidArgument(errors.New("invalid value for OCAGENT_INSECURE"))
+	}
+
+	return ocagent.NewExporter(agentOpts...)
+}