Add functionality to add old buckets to etcd on startup

Buckets already present on a Minio server before it joins a
bucket federated deployment will now be added to etcd during
startup. In case of a bucket name collision, admin is informed
via Minio server console message.

Added configuration migration for configuration stored in etcd
backend.

Also, environment variables are updated and ListBucket path style
request is no longer forwarded.

cmd/bucket-handlers.go

@@ -17,8 +17,10 @@
 package cmd
 
 import (
+	"context"
 	"encoding/base64"
 	"encoding/xml"
+	"fmt"
 	"io"
 	"net"
 	"net/http"
@@ -34,8 +36,52 @@ import (
 	"github.com/minio/minio/pkg/event"
 	"github.com/minio/minio/pkg/hash"
 	"github.com/minio/minio/pkg/policy"
+	"github.com/minio/minio/pkg/sync/errgroup"
 )
 
+// Check if there are buckets on server without corresponding entry in etcd backend and
+// make entries. Here is the general flow
+// - Range over all the available buckets
+// - Check if a bucket has an entry in etcd backend
+// -- If no, make an entry
+// -- If yes, check if the IP of entry matches local IP. This means entry is for this instance.
+// -- If IP of the entry doesn't match, this means entry is for another instance. Log an error to console.
+func initFederatorBackend(objLayer ObjectLayer) {
+	// List all buckets
+	b, err := objLayer.ListBuckets(context.Background())
+	if err != nil {
+		logger.LogIf(context.Background(), err)
+		return
+	}
+
+	g := errgroup.WithNErrs(len(b))
+	for index := range b {
+		index := index
+		g.Go(func() error {
+			r, gerr := globalDNSConfig.Get(b[index].Name)
+			if gerr != nil {
+				if client.IsKeyNotFound(gerr) {
+					// Make a new entry
+					return globalDNSConfig.Put(b[index].Name)
+				}
+				return gerr
+			}
+			if r.Host != globalDomainIP {
+				// Log error that entry already present for different host
+				return fmt.Errorf("Unable to add bucket DNS entry for bucket %s, an entry exists for the same bucket. Use %s to access the bucket, or rename it to a unique value", b[index].Name, globalDomainIP)
+			}
+			return nil
+		}, index)
+	}
+
+	for _, err := range g.Wait() {
+		if err != nil {
+			logger.LogIf(context.Background(), err)
+			return
+		}
+	}
+}
+
 // GetBucketLocationHandler - GET Bucket location.
 // -------------------------
 // This operation returns bucket location.
@@ -158,7 +204,6 @@ func (api objectAPIHandlers) ListBucketsHandler(w http.ResponseWriter, r *http.R
 		writeErrorResponse(w, s3Error, r.URL)
 		return
 	}
-
 	// If etcd, dns federation configured list buckets from etcd.
 	var bucketsInfo []BucketInfo
 	if globalDNSConfig != nil {