Update all plugins to use plugin/pkg/log (#1694)

* Update all plugins to use plugin/pkg/log

I wish this could have been done with sed. Alas manually changed all
callers to use the new plugin/pkg/log package.

* Error -> Info

* Add docs to debug plugin as well

plugin/pkg/healthcheck/healthcheck.go

@@ -3,13 +3,14 @@ package healthcheck
 import (
 	"io"
 	"io/ioutil"
-	"log"
 	"net"
 	"net/http"
 	"net/url"
 	"sync"
 	"sync/atomic"
 	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/log"
 )
 
 // UpstreamHostDownFunc can be used to customize how Down behaves.
@@ -122,7 +123,7 @@ func (uh *UpstreamHost) HealthCheckURL() {
 	}()
 
 	if err != nil {
-		log.Printf("[WARNING] Host %s health check probe failed: %v", uh.Name, err)
+		log.Warningf("Host %s health check probe failed: %v", uh.Name, err)
 		atomic.AddInt32(&uh.Fails, 1)
 		return
 	}
@@ -132,7 +133,7 @@ func (uh *UpstreamHost) HealthCheckURL() {
 		r.Body.Close()
 
 		if r.StatusCode < 200 || r.StatusCode >= 400 {
-			log.Printf("[WARNING] Host %s health check returned HTTP code %d", uh.Name, r.StatusCode)
+			log.Warningf("Host %s health check returned HTTP code %d", uh.Name, r.StatusCode)
 			atomic.AddInt32(&uh.Fails, 1)
 			return
 		}

plugin/pkg/healthcheck/policy.go

@@ -1,9 +1,10 @@
 package healthcheck
 
 import (
-	"log"
 	"math/rand"
 	"sync/atomic"
+
+	"github.com/coredns/coredns/plugin/pkg/log"
 )
 
 var (
@@ -64,7 +65,7 @@ type Spray struct{}
 func (r *Spray) Select(pool HostPool) *UpstreamHost {
 	rnd := rand.Int() % len(pool)
 	randHost := pool[rnd]
-	log.Printf("[WARNING] All hosts reported as down, spraying to target: %s", randHost.Name)
+	log.Warningf("All hosts reported as down, spraying to target: %s", randHost.Name)
 	return randHost
 }
 

plugin/pkg/healthcheck/policy_test.go

@@ -1,8 +1,6 @@
 package healthcheck
 
 import (
-	"io/ioutil"
-	"log"
 	"net/http"
 	"net/http/httptest"
 	"os"
@@ -13,8 +11,6 @@ import (
 var workableServer *httptest.Server
 
 func TestMain(m *testing.M) {
-	log.SetOutput(ioutil.Discard)
-
 	workableServer = httptest.NewServer(http.HandlerFunc(
 		func(w http.ResponseWriter, r *http.Request) {
 			// do nothing