Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/health/health_test.go

@@ -0,0 +1,47 @@
+package health
+
+// TODO(miek): enable again if plugin gets health check.
+/*
+func TestHealth(t *testing.T) {
+	h := health{Addr: ":0"}
+	h.h = append(h.h, &erratic.Erratic{})
+
+	if err := h.Startup(); err != nil {
+		t.Fatalf("Unable to startup the health server: %v", err)
+	}
+	defer h.Shutdown()
+
+	// Reconstruct the http address based on the port allocated by operating system.
+	address := fmt.Sprintf("http://%s%s", h.ln.Addr().String(), path)
+
+	// Norhing set should be unhealthy
+	response, err := http.Get(address)
+	if err != nil {
+		t.Fatalf("Unable to query %s: %v", address, err)
+	}
+	if response.StatusCode != 503 {
+		t.Errorf("Invalid status code: expecting '503', got '%d'", response.StatusCode)
+	}
+	response.Body.Close()
+
+	// Make healthy
+	h.Poll()
+
+	response, err = http.Get(address)
+	if err != nil {
+		t.Fatalf("Unable to query %s: %v", address, err)
+	}
+	if response.StatusCode != 200 {
+		t.Errorf("Invalid status code: expecting '200', got '%d'", response.StatusCode)
+	}
+	content, err := ioutil.ReadAll(response.Body)
+	if err != nil {
+		t.Fatalf("Unable to get response body from %s: %v", address, err)
+	}
+	response.Body.Close()
+
+	if string(content) != ok {
+		t.Errorf("Invalid response body: expecting 'OK', got '%s'", string(content))
+	}
+}
+*/