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/register_test.go

@@ -0,0 +1,45 @@
+package opencensus
+
+import (
+	"testing"
+
+	"github.com/cpuguy83/strongerrors"
+
+	"go.opencensus.io/trace"
+)
+
+func TestGetTracingExporter(t *testing.T) {
+	defer delete(tracingExporters, "mock")
+
+	mockExporterFn := func(_ TracingExporterOptions) (trace.Exporter, error) {
+		return nil, nil
+	}
+
+	_, err := GetTracingExporter("notexist", TracingExporterOptions{})
+	if !strongerrors.IsNotFound(err) {
+		t.Fatalf("expected not found error, got: %v", err)
+	}
+
+	RegisterTracingExporter("mock", mockExporterFn)
+
+	if _, err := GetTracingExporter("mock", TracingExporterOptions{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestAvailableExporters(t *testing.T) {
+	defer delete(tracingExporters, "mock")
+
+	mockExporterFn := func(_ TracingExporterOptions) (trace.Exporter, error) {
+		return nil, nil
+	}
+	RegisterTracingExporter("mock", mockExporterFn)
+
+	for _, e := range AvailableTraceExporters() {
+		if e == "mock" {
+			return
+		}
+	}
+
+	t.Fatal("could not find mock exporter in list of registered exporters")
+}