Move util.StringSet into its own package

A lot of packages use StringSet, but they don't use anything else from
the util package. Moving StringSet into another package will shrink
their dependency trees significantly.

pkg/util/bandwidth/linux.go

@@ -25,8 +25,8 @@ import (
 	"strings"
 
 	"k8s.io/kubernetes/pkg/api/resource"
-	"k8s.io/kubernetes/pkg/util"
 	"k8s.io/kubernetes/pkg/util/exec"
+	"k8s.io/kubernetes/pkg/util/sets"
 
 	"github.com/golang/glog"
 )
@@ -65,7 +65,7 @@ func (t *tcShaper) nextClassID() (int, error) {
 	}
 
 	scanner := bufio.NewScanner(bytes.NewBuffer(data))
-	classes := util.StringSet{}
+	classes := sets.String{}
 	for scanner.Scan() {
 		line := strings.TrimSpace(scanner.Text())
 		// skip empty lines