vSphere: set vCenter client UserAgent

Setting the client UserAgent makes it easier to identify vCenter sessions
used by the vSphere Cloud Provider.  This is useful to remove sessions that
have leaked, such as when a VCP process goes away without calling Logout().
And to test that VCP properly re-authenticates when a session is removed.

Example use:
  govc session.ls | grep kubernetes-cloudprovider | awk '{print $1}' | xargs -n1 govc session.rm

pkg/cloudprovider/providers/vsphere/vclib/connection.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"crypto/tls"
 	"encoding/pem"
+	"fmt"
 	"net"
 	neturl "net/url"
 	"sync"
@@ -29,6 +30,7 @@ import (
 	"github.com/vmware/govmomi/sts"
 	"github.com/vmware/govmomi/vim25"
 	"github.com/vmware/govmomi/vim25/soap"
+	"k8s.io/kubernetes/pkg/version"
 )
 
 // VSphereConnection contains information for connecting to vCenter
@@ -179,6 +181,9 @@ func (connection *VSphereConnection) NewClient(ctx context.Context) (*vim25.Clie
 		return nil, err
 	}
 
+	k8sVersion := version.Get().GitVersion
+	client.UserAgent = fmt.Sprintf("kubernetes-cloudprovider/%s", k8sVersion)
+
 	err = connection.login(ctx, client)
 	if err != nil {
 		return nil, err